As a jungle I can see some mistake in your speech ... and Iwill point it out so you can see where you can improve in my opinion :

I usually start red and then gank top immediately

you can't just do that all the time you have to adapt your route to your team, the other team and the other jungler

Solutions :

• Identify the lane that you think /will be pushed fast/is strong early/could snowball/ and try to finish your route near this lane (lvl 3) and gank this lane.
• Think of what route the opposite jungler will take and /couter jungle/Counter gank/ him (Lee sin is very effective for that)
• ...

I build assassin Lee Sin and build full AD, with the occasional tanky item.

• Adapt your build to the enemy team and the needs of your team

even if you prefer building AD you can just no brain build.

usually have a positive k/d ratio

It doesn't matter not your job ;-)

I can't gank them because they can get under their turret very quickly.

If they are under their tower they should be dominated

You could try going in their back ward jump Q > R > Q so you get them out of their tower and you follow them out of turret range with the resonating strike (just an idea)

If you can get them out of their tower tank 3 tower shot and leave your laner take the kill it would be a very good play

I feel I can't do anything late game in team fights either

Your role as a Lee Sin in team fight could be :

• Engage and kick 1 carry out of their team and then instant kill him
• Kick someone anoying away from your carries (the most important)
• Slow the peoples that chase your carries
• Shield your carries
• CC the other carries to help yours kill them

At a point I just backdoor, but the opposing team usually sees me and ganks me, leaving me dead to watch my team lose.

Stop dooing that... stay with your team.

I can never get my items unless I take all of my teams laning kills.

Stop thinking like that and imagine what your carries could do with this gold if you had give them the kill(s)

To conclude, as a jungle your role is not to carry your team ... It's to carry your carries so they can carry the team ;-) Most of the champion are a lot more gold efficient than lee sin... So I think you should try to play FOR your team or change your position to Lee Sin Mid.

To answer your original question:

Clearing waves is bad in some cases.

By just clearing the current wave, you are allowing the minions on your team to push closer to their turret. If their champion is capable of freezing the lane, this will put your buddy at a disadvantageous position if the wave you cleared was in the middle of the lane.

In general, you want to do one of the following if both are dead:

• Last hit the current wave or simply leave. (If the lane is neutral)
• Push the wave up to the tower and make the tower kill one or two of your waves. (This is to reset the lane, the closer minions are to pushing into an enemy tower, the more priority you should give this.)
• Clear part of a wave so they don't push into your own tower. (To prevent resetting the lane in the enemy's favour)

In other words, it depends on the situation, but a general rule is that you want to keep the lane as close to your side of the map as possible, and as the game drags on (if you are winning) you want to shift that balance from your side to their side more and more, so you can start putting pressure and forcing down turrets.

If the enemy is alive:

You either want to:

• Keep the lane so that you don't lose a turret as fast. (This is mostly true for strong pushing champs)*

• Leave the lane if the enemy is weak at pushing if you can do more beneficial stuff. (Get / give buffs, secure dragon, gank another lane, whatever)

• Kill the enemy champ. (If you think you can do so)

*Note: In some cases it is recommended to let your tower get destroyed, if the player who is laning against an enemy is performing really bad. Letting the tower get destroyed allows the minions to get pushed into your side of the map even further, allowing the bad laner to freeze his lane at the second turret and farm there. (Of course, you should pay attention at this point because otherwise you're just giving away turrets)

If teammate is alive but enemy is dead:

Either:

• Leave and let him continue farming.
• Help him push down the turret.*
• (Situational, mostly for bot-ganks) Kill Dragon

*: Pushing down turrets at botlane can be especially helpful as it allows you to pressure them more, thus giving you more leeway to take dragon out, as they can't permit to stay close to it if they don't maintain vision of the river and jungle 24/7.

Also, if there are other lanes that you can gank right away, you should of course do that instead of just farming minions for your buddy. A few minions aren't worth as much as another kill + assist.

And as a closing note: Ultimately it also sort of depends on the champion composition. Sometimes you have a team (or jungler) that is capable of pressuring a lot more than the enemy team, in which case again, you want to push up a bit more so you can take more control of the map, sooner.