Refrigerator and freezer are not cooling properly

Hi, I have a GE refrigerator/freezer which has the freezer at the bottom as a drawer and the refrigerator is at the top. The refrigerator temperature rose to 55 F and the freezer temperature rose to 40 F when I noticed the problem. I removed items from the refrigerator to look at the vent that allows airflow from the freezer to the refrigerator and noticed it was blocked by a easily movable "flap." I opened up the flap and waited to see if the temperatures would improve. After about 20 hours, the temperature steadily approached normal temperature (37 F in the refrigerator and 0 F in the freezer). However, at about 24 hours, I noticed the temperatures in both compartments were rising again (40 F in the refrigerator and 10 F in the freezer). My freezer does not have any frost or ice buildup. I turned off the machine and let it manually defrost overnight but there was not much melted water the next day. Does anyone know what could be the problem and what else I should check? Thanks!