Lipo Shim not turning on

HI All, hoping someone can help me.

I’ve got a [LipoShim] and 2 batteries, a 3.7v Lipo 1200mAh, and a 3.7 Li-ion 6600mAh.

At the time in question the 6600mAh is reading 3.92v, and the 1200mAh reading 3.82v.

plugging the 6600mAh into the LipoShim, all is well; the blue LED lights and the pi(zero w) boots as expected. Plugging the 1200mAh in the LED does not light, and the Shim makes a hi pitched whine.

I can power other devices from either battery, and my charger reports them both as in good health.

Can anyone suggest a reason that the 1200mAh might not be gettign along with the LipoShim?