The problem's usually stem from water main pressure and flow and combi output flow. Basically as Simeon says the bigger the combi the more flow. But only providing the water main can deliver the flow and pressure rates required by the combi to the combi.
So basically you have to check the main out first, to ensure the combi is getting enough of both to give its full output. If it is, then you may have the problem of having too small a combi which you can do little about. But if you check the combi output and find it is delivering enough to provide the showers requirements while overcoming any pipework resistances, then obviously the problem may be with the shower.
But just off the record its probably the combi flow rate or the main, its quite common for it to happen and although it is a nuisance the alternative is to get a bigger combi. which usually means lots of money.
