Does the Challenge have a minimum distance for qualifying rides?
If it is a RideMoreOften Challenge, sometimes there is a minimum distance for a ride to qualify. If the minimum distance is 2 miles and your ride was only 1.5 miles, it will not count towards your goal.
Are your Strava Privacy Zones interfering?
If your Privacy settings are cutting off a portion of your ride, making it too short to qualify for the Challenge, then adjust your settings (without compromising your privacy) to maximize your distance.
If your settings hide the start and end points of every ride and your ride didn't go beyond those zones, then the ride will not upload to Ride Spot or count towards Challenge goals.
For example, if I have my privacy settings set to hide the first and last mile of every ride, and I rode 1.5 miles to the store, the entire ride is inside my Privacy Zones, and would be hidden entirely and not count as a ride in Ride Spot.