could be that its farther from the router or theres some sort of intrefernace from something beween the the comp that isnt and the router.
things to try.
1. try to clear up any wire clutter around your anttana if its in the back of your comp (like a card) then try to pull the pc out from the wall some (the PSU can cause interferance sometimes.
2.place the router closer to the the comp thats having the problems.
3. try one of the many anttana hacks out there (its basicly metal like foal or steel in a U shaped pointed towards the router)
4. boot your routers out put with 3rd party firmware.
5. just get one long ass eth cord and plug right into the router