Amana defrost circuit will come on, but unit does not automatically defrost

Dear Fridge Dr., I have an Amana, bottom mount freezer model #BX21TW. First I notice frost around water inlet tube to ice maker. The freezer works fine but the refrigerator blows warm air . When the freezer door is opened then cold air is blowing into the fridge but when freezer door is closed then warm air is blowing into the fridge.

And sometimes it seems that the air is hardly blowing at all. We have unplugged the fridge and let it defrost and then plugged back in and it will run fine for a few days then starts blowing warm air again. So we think it has something to do with the defroster. The timer seems to be working properly and the heating element heats up and I can hear water sizzle as it hits the element. Do you think that maybe timer is not on long enough or that element is just not staying on? Do you think it could be the timer, heating element or even the thermostat? the fridge is only 2 years old. We can likely repair this ourselves if we know what it is, as my husband is a mechanic. Thanks for any suggestions