If the belt moves a bit from the left to the right of center, it is clearly a problem with the alignment or leveling. This can be easily fixed.
1. Define if your belt is tighter or slack. Basically no slippage between the belt and the pulley will be appearing. Also, the belt should be adapted to the crown of sprocket. Any more tension must be prevents because it could rupture the belt seam .
2. Remove the right bearing to the exterior to make the head roller advance in a position that will tighten the belt.
3. Put the left one in to move the head roller in a position that will slacken the belt.
4. Make an adjustment, and then look at the belt. Make a new adjustment and study again. Small adaptations are requiring checking if you do not overly loosen or tighten the belt because it may cause problems.
5. Reiterate the process till the belt looks centered to you.
6. Clean the belt. Any foreign things can alter the tracking.
7. Verify that the snub rollers, carry idlers, return idlers and pulleys are all parallel to each other. Every non parallel roller can modify the belt tracking.
8. Be sure they are perpendicular too to the belt center line and square with the frame. If the belt excel these tests, it basically be centered, but a final test can confirm this.
9. Launch the conveyor belt in the reverse run, to the tail pulley. The belt must be empty. Check it during three entire revolutions.
10. Re-launch the conveyor in the current direction and wait another three revolutions. Rectify the tension to center of the belt: if you tighten on the left side, the belt must moves to the right .
11. Put an entire-load on the belt and move it in reverse direction for three more belt revolutions. The machine track differently when load is added.
12. Check the conveyor move in the current direction with the same load for three final revolutions.
13. Remove an idler if the belt is over adjusted.
14. You can do that as the belt is moving.