That's an ongoing bug since years, Strava is unable to correct that. There is a thread about that but I can't find it, the gist was, you could change the activity type to another sport and then back to ride, this recalculates the power curve.

I had same problem and tried a couple of the other solutions which failed.

I then exported the GPX file and uploaded it as a manual entry. This updated my power curve, i then deleted the original activity.

Hello All, 
Thanks for the post and sorry for the delay. @Jan_Mantau is correct about this. Our team is aware of an issue with how the Best Efforts Power Curve is auto updated with new activity data. While our team works on this issue, I wanted to share a temporary work-around that will allow you to include the activity in question in your Power Curve chart:

• Navigate to the activity on Strava which is not being included in your Power Curve and choose the option to edit the activity.


• From the Edit Activity page, change the sport type on the activity to any of the available options, save your change, then edit the activity one more time to change the sport type back to the originally selected value. 

We appreciate your patience while our team addresses the underlying issue.