There are 2 possibilities that I can think of.
1 - your tamp is not quite right. If the tamp is uneven, or the coffee distribution is not right before tamping, you can get what is called channeling which can, in turn, put more liquid to one spout than the other.
2 - your PF spout might need a clean. Sometimes coffee oils and gunk can build up in the spouts and cause flow issues.
Hope that helps.