I rescued all 10 claptraps on both playthroughs on Borderlands but I only have 39 inventory slots. How can I get 42 slots for the achievement?
This happened to me too. After searching around, I’ve found that while playing in online multiplayer games, if the host has gotten a claptrap that you haven’t, there’s a good chance that it will give you a grenade mod instead. There are 2 solutions
1. Play an online game with a person who hasn’t gotten any claptraps and get them with the other person as host.(this worked for me)
2. Start a new character and play offline until you get the claptraps.(Ive done this too)
Also as a note, some achievements that are for doing challenges are also best done offline.
Hope that helps!

March 7th, 2010 at 6:40 am
Need the General Knoxx DLC:
You get a new claptrap in lockdown palace!
References :
March 7th, 2010 at 7:21 am
This happened to me too. After searching around, I’ve found that while playing in online multiplayer games, if the host has gotten a claptrap that you haven’t, there’s a good chance that it will give you a grenade mod instead. There are 2 solutions
1. Play an online game with a person who hasn’t gotten any claptraps and get them with the other person as host.(this worked for me)
2. Start a new character and play offline until you get the claptraps.(Ive done this too)
Also as a note, some achievements that are for doing challenges are also best done offline.
Hope that helps!
References :
From playing