Not sure whether this is due to KAS or KIS, but the CS-R2 Portable Struts don't have an option to set the amount in a KIS inventory like the fuel ports do, meaning you need one slot for each strut endpoint (wasting a lot of volume)

It's a known mods compatibility issue. Some mods add extra modules to the existing parts, and KIS only allows stacking if all the modules on the part are known to KIS. The stackable modules/parts are whitelisted via settings.cfg. To fix the issue in your game you need to find which mod introduces the problem, and then figure out the modules it adds.

Hmm, alright thanks.