Why don’t I get XP in some matches in BF3

You have to own more than half the points for bleeding to occur. There are usually (always?) an odd number of points, so 2/3 or 3/5 is required for bleeding of tickets.

At max "bleed" -- when the enemy owns all capture points -- based on my testing, you lose about 1 ticket every 3 or 4 seconds. It definitely hurts and frequently causes us to lose matches where the ticket count is close. It's also why I get frustrated with teams where they're more interested in fighting than making sure we don't lose all the damn capture points, but I digress..

I can't say exactly how many tickets you bleed when the enemy owns more than half but not quite all the capture points. It's less rapid, but I'll have to do a bit more testing.

As far as I have been able to work out, being assigned to the position of squad leader is a case of "first in". When you join a squad, you may notice that there is an order to it. If someone higher up than you then leaves the squad, everyone lower moves up a space. In this fashion, you can only become squad leader if you are in the second position and the squad leader leaves (you will be notified with a big heads-up "You are now the Squad Leader" notice).

Edit: It is possible to make a squad mid-battle.