Why am i only able to enchant items to 8% even though Enchanting is 100 and I have all perks

From UESP, under Bugs:

The value of enchanted equipment is inversely proportional to your current level of Enchanting. For example, a leather bracer enchanted while under the effects of a Fortify Enchanting potion will have less value than a leather bracer enchanted without a Fortify Enchanting potion. If creating enchanted items for sale, remove any Fortify Enchanting buffs you may have equipped and don't drink any Fortify Enchanting potions.

Assuming this isn't a glitch (it most likely is), it could just be a hidden mechanic to make creating the same item over and over again less profitable the more times you do it.

You can look at it one of two ways:

1. From a game design perspective, you wouldn't want players to be able to "farm" one item over and over again. This gets tedious and makes the game feel like a job.

2. From a role-playing perspective, when your first Turn Undead dagger was made, there were presumably only a few of them in the world of Skyrim. Now that you've made 86 of them, they aren't worth as much. You've turned a rare-ish magic weapon into something commonplace.

And before you shoot down #2 (because the first dagger you made is still worth more than the last one you made), it's obvious that the inhabitants of Skyrim will pay more for one of your early works to proudly display in their shops.

It sounds like your Enchanting skill is too low. Under the Enchanting skill tree, the first perk is called Enchanter. As you spend perk points in Enchanter, it will increase the power of your new enchantments. To get your new enchantments to double in strength, you'll first need to get your enchanting up to 80, spending perk points on the way. Once you have Enchanter at 5/5, your new enchantments will be twice as strong. For example, when I first placed Fortify Archery on an item, it would only fortify it by 8%. Now, and I'm only at 4/5 Enchanter (80% stronger enchantments), when I do the same enchant, it fortifies by 32%.

In addition to Enchanter, you can spend perks in other Enchanting skills such as "Insightful Enchanter" which will increase Skill enchantments on armor by an addition 25%. There are others that will boost your fire/frost/storm enchantments, as well as health/magicka/stamina boosting enchantments, by 25%. These do stack with enchanter for a total of 125% stronger enchantments when Enchanter is 5/5 and the necessary perks are acquired.