Sounds like it could be working OK.
the Combi can only put out what it gets in, the fan speed is dependent on the amount of heat in the unit that it is trying to dissipate.
on electric, you will have only 900 or 1800w (depending on the setting) whereas on gas you will be using upto 4000w on a Combi 4 or 6000w on a Combi 6, so a significant difference. Basically this is the reason for the difference in the fan speeds. If the fan spun fast with little heat generation behind it, the fan would be trying to blow the air out before it had got warm....so blowing cool air rather than hot.