well you can have the same plant react differently... heavy eater as an example...
I doubt its the gnats... they are more just of a nuisance... ive had gnats threw the whole go... its like what budelee said the soils too moist allowing them to breed and hatch.. which also is my guess at why the plants not happy...
you could always try and break up the soil so it dry's .. but u could be scared of hurting its roots... only other thing I can say is go get papertowel and line the top of the soil with it... u'll see it absorb