No increasing the amount of water will not give more pressure.
1, size of pipe to bath min (22mm0
2, does it have any resrictions (ie, ball-o-fix valves) or lots of elbows
3, is any pipe work likely to air lock (high pints in loft etc)
If all these are OK! then raising the tank will be the answer, you should have min 1200mm from the Base of the tank to the shower rose on the wall.
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