Add coded value into the domain - schema lock?!

Hi,

I have just created a simple tool for inserting a value into the GDB coded value domain (I want to give users this possibility using GP service on the web) but I get error about a schema lock. Well, on data from this GDB several map services are being started BUT I can add any value to the same domain in ArcCatalog/ArcMap without problems. Why GP tool can not do this?

Thanks,
Bart