Hi,
The other day I was cycling a round in Watopia and in the end, I was notified that my FTP was increased. YAY! A week + later I did the same round and tried to keep my power 10Watt higher than my FTP. I thought I was successful but there was no notification of an increase.
Then I released that I did was quicker and did my round in under the 20 min. Can that be it? For FTP to increase, do you have to maintain the power for at least 20 minutes? Or can you increase in a 15min ride too?
Yeah, I think you are correct. Needs to be at least 20 minutes.
Also note that in order to increase your FTP you need to maintain a higher power than your max 20 minute power, not just more than FTP.
FTP is an estimate of what you can put out for an hour based on extrapolating out what your power output was for 20 full minutes. Just doing 20 minutes over FTP isn’t going to budge your number. Ex. my last FTP test I put out around 255 for the 20 minutes but it raised my FTP to 242.
FTP is calculated by multiplying your best 20 min power by 0.95.
255w * 0.95 = 242.25w FTP
That’s why the FTP did not improve… I did the round in under 20 minutes, next time cycle on for a little while. 
Also if you just go all out with fresh legs, you will probably get an estimate that you most probably can’t hold for an hour
I don’t think I can hold 95% of my 20 min power for an hour.
I don’t see a good reason why Zwift doesn’t just use 20 min power and forget about multiplying by 0.95.
Possibly because the accepted standard definition of FTP is the average power output you can hold for an hour.
A common estimate of FTP is 95% of the power you can hold for 20 minutes, but it’s just an estimate.