Amplifiers will only amplify the signal they receive. If the signal is poor, you will experience no gain. (Play on words there).
Anyway.
As mentioned above, you could try an attenuator on the input... this will help if the signal is 'too strong'. Unlikely with a Digital Broadcast Signal, but not unheard of.
If it's not too much of a ball-ache, I'd start by upgrading to the best Coaxial cable you can find, thats if you aren't already using Satellite grade stuff. (you dont get much better than that (even if specific manufacturers say so)).
The splitters you are using are the correct type (for non amplified), not the piss poor triangular things which have losses greater than that of Northern Rock.
Check and double check your coaxial fittings. I'd be inclined to remove them all, even if they dont look suspect and fit new ones, ideally with a proper crimping tool.
All of this would be easily cleared up if you knew someone with a signal tester.
