Just replaced a battery in a friends Tag Formula 1 Alarm. The alarm now works but seconds hand not ticking. The watch had been sitting for a long time in a drawer.

The battery that I removed was a Renata 396 but I replaced with a Renata 397 which is supposed to be compatible (it’s just a high drain version). I’m wondering what the purpose is of the little thin plastic cover between the movement and caseback is and perhaps I haven’t put it back correctly.

Any advice or perhaps the battery isn’t compatible?

Photos to show what I’m talking about: