I would say this is definitely a calibration issue. If you can average over 30km/h outside, then your average power should be much higher. I imagine at least double.
Changing ftp or trainer difficulty on Zwift will have no effect. Your speed is only a calculation from power, so nothing you can do there. Cadence measurement is useful, but will not affect anything else.
The Kickr should be accurate once everything is set up, but I saw this on Bikeradar:
## Wahoo Kickr Snap power measurement
On the first test unit we had the power numbers kept dropping out every few seconds, regardless of software used. We’d see a reading for a few seconds, then 0, then a reading, then 0, etc. Wahoo exchanged the unit and the replacement did not have this issue.
In initial testing, we found power measurement to be off by 40–80 watts, compared to reliable power meters, after completing Wahoo’s spindown calibration. In our initial review published 11 January 11 2016, we called the Snap’s power measurement lousy. Turns out, the situation is a bit more complicated.
For the Snap to correctly measure power, two factors must be considered.
One is the rolling resistance, which includes the tire and the flywheel’s inertia. This is what the standard spindown measures and accounts for.
The second is the electromagnetic brake. Wahoo CEO Chip Hawkins said the unit’s algorithm for power measurement is based on the pilot batch of Snap trainers — and that subsequent batches of Snaps had a different factory configuration, something Wahoo only discovered after the fact. Thus, the advanced spindown must be completed by users to measure and calibrate for the unit’s brake in order to get accurate readings. This only needs to be done once, but it does require the latest version of the Wahoo app.
Also Shane Miller AKA GP Lama has a good video (with more links too) https://www.youtube.com/watch?v=waV9l2wMANM
Hope you can get it sorted, Zwift is not so fun if your power is not accurate!