Some of us have experimented with filling melodicas with water, sparkcle. I have done it with a couple of Suzukis, though not with the M37. Whatever you do is at your own risk, but here is what I would do in your situation. I would fill the melodica with warm water, holding it vertically and pouring into the mouthpiece hole. I would then blow into it and work all the keys. You can do this alternately with the spit valve open and also with it closed. Water will come out of the keyboard. Work most of the water out, repeating this process 2-3 times and then continuing to work the keys and blow into the melodica getting as much water out as you can. At first the keys won’t play, but eventually all of them will play. You may also spin the melodica around with your extended arm, holding it by one end and then the other. Then store it on end with the mouthpiece hole down. It may take a couple of days for it to get dried out to where it plays normally. I have had no subsequent problems with the melodicas that I’ve done this to. If this doesn’t eliminate the sticking, I’d try soaking the melodica in water (or 50/50 mixture of water and 3% hydrogen peroxide), for an hour or two.
It sounds to me like your problem is not from lack of cleaning the inside of the melodica. It’s more likely from a lot of dust in the air getting into your keyboard or from something being spilled on the keyboard. Some sticky material has gotten in between the keys.