new 2gb mem module won't work with old 512mb mem module

If you have a Core Duo then 2gb is the max that will work.

Memory issues can sometimes show up in the interaction between modules.

Assuming that the ram works when in each slot - ie the slot does work, then I'd blame the ram. Not all modules are created equal and it's pretty typical of faulty stick that it doesn't play well with others. I know that some cheaper ram doesn't work well. Kingston's Value Ram range for example.

I'd recommend returning the ram and getting one from Crucial (made by Micron who supply Apple Crucial.com) or Samsung, Hynix, Nanya (harder to get). These are all the ones supplied by Apple as standard.

You could also try Kingston (not value ram) or Transcend as I understand they work well too.

BUT * If you have a Core Duo MacBook Pro - NOT a Core 2 Duo then 2gb is your maximum. In which case you're actually better off getting 2 x 1gb so you get the minor advantage of dual channel ram (2x1gb is better than 1x2gb) and 1gb ram sodimms are dirt cheap at the moment.

Best of luck.

Wrong info on my part
Stedman

Message was edited to remove my incorrect info by: stedman1

Your signature says that you have a Core Duo MBP. 2 GB is the maximum amount of RAM supported, as Rajesh Sharma pointed out (so you MBP won't start with 2.5 GB). That's normal behavior.