Firstly, the BKool trainers do not need any calibration, since the mechanism works in such a way that the pressure of you sitting on the bike applies the pressure to the roller (as on the road), rather than other A-frame trainers where you need to tighten the roller up to the wheel. So it can’t be that.
I have a BKool Pro and use it without issue on Zwift. It is the latest version, and I made sure the firmware was up to date and I’d set it up in Ant FE-C mode (via the BKool software). There are some forum entries on how to set up your BKool Pro around here. I know there are some issues in workout mode when ERG is set, so look around for that setting to see if it maybe impacts your ride.
I’ve had similar issues before with other brands of trainer, and I found that either restarting everything (yes, switch it off and on again works sometimes) or moving the Ant receiver closer to the trainer sorts it out. It could be noise interference or something. I would also turn off any communication protocols that you are not using, eg: bluetooth on your PC or phone.
Another thing to check is whether your cadence is paired in Zwift too and reading the correct value (I assume it is as you mention you were doing 90rpm)