Make the game free. Or at least to where you can play for 1 to 2 hours free per day, before you can't anymore, and you have to pay.
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They used to have unlimited play. But then obviously had to reduce that because of the cost from google. So they reduced it, then reduced it again, and again.....and all that happened was, their player base size went up, but most free players didnt transfer over to paying players. All they did was created multiple accounts, and gamed the system to still technically play unlimited.
I dont know if this stat is accurate. But it was stated before that for every 160 free games that was played, they lost a $1 because of googles fees. You times that by tens of thousands of games a day, and they would have to start charging the paying players $150 a month to offset those losses.
So in closing, free games is never ever ever coming back. Even if Google were to reduce their rates some, its not coming back. Even if google reduced their rates a bunch. Its not coming back.