many possible things.
(1) too small for this route. When you're flying too far for a small aircraft(like 3000NM for a 737), the bigger the demand, the bigger the penalty. So LF decreases while demand increases. Probably not your case(decrease in LF is not that quick).
(2) Price war. Opponent decreases prices on that line
(3) more opponents flights on the line (if potential is 200, and your opponent makes 3 flights, your base potential is 50)
(4) better seating for the opponents - especially on the longer routes.
(5) opponent flight at the same time. If you fly at 6, your opponent at 6, 12, 18, the potential will be divided in three for each starting time... and then the 2 flights at 6 are divided in two.
(and many others)