Ok, if we are letting it all out then -

In ANH C3PO is kind of an asshole. He clearly did not give a fudge about R2 when Luke was taking him home together with that other droid. After all they've been through he just looked back at R2 and didn't say ANYTHING, he just looked away. Like, "Sucks to be you, bro! I guess I'm lucky!".

You can say, "But, hey, when that red droid broke down C3PO recommended to take R2 instead!", but that's bullcrap! He should have said "I ain't going anywhere with my techno-friend!" as soon as Uncle Owen decided to take him.

But instead - he had a chance to flee from Jawas and he took it and left his best friend behind - alone in a desert, kidnapped by a bunch of weird little Sidiouses.

What a douche!

But then later he tries to give an impression that he's friends with R2 - he tells Luke how they both survived Clone Wars and then he's worried about R2 not coming back from the space fight over Yavin... It doesn't make any sense!

It's not exactly a big deal and I could come up with my own explanation why C3PO didn't act when he had to, but it's just something to consider.