I'd start by following the steps on this page, specifically steps 3 & 4 - https://support.strava.com/hc/en-us/articles/216918967-Troubleshooting-GPS-Issues

My guess is that your phone is going into a power saving mode because Strava isn't allowed to use battery (and/or GPS) in the background.

Problem solved. I think restarting the phone have helped but anyway this steps from the link helps.  Thank you for your support