Often when you switch screens away from the Zwift mobile app, when you return to the app, you cannot activate power ups from mobile app. You can activate with computer space bar and then next power up works fine.
Hello K C.
Sorry to hear that. Can you tell us what iOS app version are you using?
On a different note, a brand new app version (v1.2.1) is now available at App Store. Please update your current app [if you have not done it]. Let us know if this fix the issue.
This problem happens on the newest version of your mobile app.