every time i level my block % decreases. what gives?

dexterity

yes i know. but why is that if it's at max block?

DEX needed for max block = {(150 * clvl) / blocking} + 15, rounded up.

so the more the clvl, the more dex you need

if it didn't depend on your lvl, it'd be impossible to get max block on low lvl, or way to easy to get mb on high lvl

Your block rate is relative to your character level. Also different monsters have a higher chance to hit you then others. Like Mephisto compared to a zombie. As you level you may need to put a couple points into dex to maintain it at 75%.

You can also google "diablo 2 max block rate calculator". You plug in your level, current block rate, and dexterity and in return it will tell you how much dexterity you need to achieve max block.