Is it a bug??

It's happening to me all the time. Whenever I used to put my pokemon in different gyms, say I was able to put 4 pokemons in 4 different gyms this morning, the next morning if they return, I would be awarded coins for only 1 or 2 mon. I am free to play player and need coins to upgrade my storages and buy incubators, but at this rate, I don't think that I would be able to do so. An example screenshot is also there.