Magic Mouse 2 and Bluetooth mouse clicks stop responding on iMac

Apps and websites stop responding to magic mouse2 clicks (standard L clicks that select, not open a menu) - issue develops with use over a couple hours, sometimes sooner. Happens with a new magic mouse2 and another bluetooth mouse. I've tried the AI suggestions WRT settings (mouse keys, etc.) and plugging the misbehaving mouse into connection cable (to Mac) while on - supposedly this resets things. Nothing has solved the problem. iMac 2019 Sequoia 15.7.9 magic mice 2 A1657 A3204. I haven't reset the SMC as this isn't a power issue. Ideas?