As others have said, I've never had a problem with them.
As such, we seem to have two groups here: Ones who constantly have problems and those that don't.
As such, for those that have problems, can you please explain HOW you attempt to open these?
I use the method 230RN suggests - rather than just pulling, I pick an end and give a steady rolling pull so that it releases the seal under the tab, then expands that. I think that if one just pulls, you're putting force on the entire seal, and that may be beyond the strength of the plastic tab.