Make sure you did not install any of the parts for in sump use like the 9010.400 sleeve below cover. You might try raising it very slightly if it is making any sort of gurgling noises when the cup is installed, this would indicate water is coming in through the air line. I would also make sure the air adjustment is open at least 3 turns and not more than 5. Too much air can be just as detrimental to good skimming as not enough.
If the canopy is really enclosed we know from our own tests that humid air with high composition of certain gasses from decomposition will inhibit skimming. This is the only way I believe the warmth of the light could cause an issue. If it is a really sealed hood that is filling with humid hot air with higher concentrations of methane, CO and CO2 and nitrogen gasses it will knock down the skimming power.