Normal AA or AAA battery, or a CR<something> button? If it is a button cell, try wiping it with alcohol before you insert it. Many modern button cells have a coating that tastes horrible to prevent people (children) from taking them into the mouth. I’ve noticed that on some brands, this coating measurably increases the resistance of the cell, not only reducing the the power it can deliver, but als making it appear “emptier” than they are ofor the measuring circuits. Wiping off the coating with alcohol (90% isopropanol) expanded the cells’ life (in a kitchen scale to boot!) significantly.
Could be. Key fobs use button cells.