Hi Richard
Microgaming have some great slots games and they just recently launched Tomb Raider – Secret of the Sword. TR is a 5 reel, 30 pay-line where players are rewarded with free spins 
(kinda went a bit of track)
Have you analyzed why the MG casino is not converting well for you? My questions for you are how are you promoting the microgaming casino(s)? What type of landing pages are being used? Is the player offering competitive to players? Then compare your analysis to a casino that is coverting well on your site and note similarities and the differences.
With your differences speak with your affiliate manager of the program that is not converting to see if you can get something similar to what is converting with your other programs.
We hope this information helps