Your pump isn't making any strange noise right? If so, maybe the impeller is clogged and so rotating at a lower speed. Your pump isn't one of those with 2 velocities right? Tension is ok? No air can be seen through the lid? You don't have any type of vaccumming devices connected? If all that is OK, I think you would have to contact a plumber, but you could also try this;
turn off your pump
take out the lid
insert your garden hose through the hole that is above the basket (pipe where water comming from the pool enters while pump is on) with something as a cloth covering all the hole as to prevent a loss in pressure
turn the hose on and let 1 of the valves opened an the other ones closed (you will have to try with each one then)
see if the "backflow" clears the line, or if you notice that the line isn't clogged (if water comes out from the skimmers for instance with the same flow as the hose), try opening another valve
if nothing listed above works, better calling a plumber.