Not sure what yours looks like, but i use some of those really cheap plastic ones, with a pump type thing on one end... normally when i seem to start losing suction, its because the pump area might have gotten a small hole etc and causes the siphon to not work as well...
I've noticed in the past on some of my old ones, the holes can be quite small and hard to spot, so i'm guessing that maybe a possibility?
If the thin tubing has even a tiny gap where it fits onto the thicker gravel part, you will loose suction.
Some have a check valve of sorts, in the section where the small tubing and larger part meet, i this is malfunctioning you could loose suction.