Ok Dave, I'm no expert but I'll have a go.
1) Are the bulbs the same - same ID markings on them?
2) Did you bend the bulb housing slightly when prying out the old bulb?
3) Was the bulb so old that it has been re-manufactured to new specs - including the requirement of a new (updated) bulb holder?
I'm unsure as to how you might find this last bit out aside from taking a guess determined by how long ago you changed the bulb last time.
I'm pretty sure you could go out and get a new bulb holder to suit your new bulb. Someone on CPF should be able to point you in the right direction and also assist you with walking thru the modding process.
Best of luck - I hope my little theories help you Dave!
Cheers,
Tim.