Cant build in Plotworld

Hello,

my problem is, that no one can build on his own plot in my plotworld. I'm using plotquared and I have wolrdguard installed. Everytime a user without OP tries to breake a block on his claimed plot, it gives him the WorldGuard-Deny-Message. (Hey! You cant break that block here!) I tried a couple of things (Setting the spawnprotection=0 in the server.properties, checked global worldguard flags), but all these arent working.
The only protection-plugin that I'm using is worldguard.

Thanks in advance
Gigamaster