First, check the skimmer & pump basket to make sure they are not full of debris.
Next, check to make sure your filter has been properly back washed or cartridges cleaned. Try bypassing the filter on recirculate, or remove the cartridge.
If both of those steps have been eliminated, check for air leaks around the pump lid, and check the pump impeller. This is the part in the pump that is connected to the motor that moves the water. Impellers will clog from time to time from small debris.
To inspect the impeller you will need to open the pump to see inside the housing or volute.
Check your pumps manual for a parts diagram to see how the pump comes apart.