I changed the battery to my armitron now watch and can't get the back on. any suggestions?

A replacement battery has be the compartible brand serial number to the watch may be new howevr least capable of displaying an functionality. Second, the new battery should not have been mixed up with the old in the process of performing the changes else the wrong one was returned into the watch. Reinspect the old and new batteries. Third, the battery has to be firmly placed into the socket area before current supply runs through the watch, perform an shuffle around within the socket to acquire is the new battery placement was least in firm grip at placement. Refering to the watch user guide on the placement of batteries to the watch is useful. Finally, consult with technician when necessary.