It could be caused from a number of different things; here is a list of possible causes, and remedies. Excess water usage. — reduce the water use and see if situation returns to normal. Improper plumbing — lines are not at proper slope and could have a hump in them. Blockage in plumbing — grease and other things not to be used in system can get hung up on pipe joints Frozen lines — not enough water usage, disturbing snow cover on lines, improper installation of piping Pump failure — check pump, floats and if not working, replace with same Type and Model Roots clogging pipes — Have line Roto Routed, check line for cracks, broken joints and remove roots next to it