Following on from Kev_UK,
turning up the gain can cause problems in itself, the gain could also increase any noise on the line. 56K is the absolute maximum the modem can handle and very rarely will it operate at that speed. Line, ISP, PC etc all being factors in the equation.
There is also the possibility you could be on a DACS (digital audio carrier system). It can send two telephone calls down 1 pair of wires, used when there is insufficient line plant(wires between you and the exchange). This will slow you speed down conciderably but you could be completly unaware of it otherwise.