Why does my calor gas combi boiler only sometimes feed water to my shower?

So frustrating. never had a combi boiler with calor gas before. I am a hopeless woman alone and broke. (had to run from a violent husband and leave everything I own) Central heating works fine. The bathroom sink has hot water. The bath sometimes does. But more often it does not. When I need a shower in the morning or anytime it seems..no water.. bath takes for ever to fill from a tiny trickle of water..the guy who rents the place to me showed me about turning two screws to put a red needle on top of a white one. shower on a tap system, pull middle bit up to push water up shower pipe..PLEASE CAN ANYONE HELP???? says eurocombi styx on it. it has ariston written on it in pen. i have had great shower in it..but why not all the time???