Wireless boosters can only boost the signal they receive. The booster needs to be sat somewhere where the signal is known to be good, so it can then 'throw' it a bit further. If you site it next to the computer, you already know that the signal is poor there, so the booster doesn't have much to work with. Does this make sense?
Hard wire cabling is much better - or move your server.
I moved mine into the hallway, to get a better signal upstairs. It still works downstairs.