Need to back up a bit:

1) is the bike new or old? If it's old, grab a magnet and find out if those rims are steel
2) Which brake are you using? If you're using exclusively rear brake to stop, especially downhill, things might get scary.
3) Ensure they're adjusted correctly. Look at the pad surface and make sure it's contacting the rim well - ensure it's not missing the rim, or tilted so only an edge is touching. Also, very important - make SURE the pad doesn't rub the tire.
4) As mentioned, check those fenders to make sure they're not stopping the brake.

Other than that, you've used enough pads to ensure that they're probably not at fault. Though don't go back to the Tektro, those pads aren't very good.