The trouble your having is your going the wrong direction with having flat pulleys/rollers. I run a lot of conveyors for fertilizer and corn. You tighten the side you want the belt to move away from. Think about the rollers being a wedge. The belt will move to the lower/shorter side. The wider the belt the harder it is to get straight. A little bit will usually move the belt when your close to straight.
Older flat belt driven shellers/grinders and such, just about all had domed pulleys. They are more forgiving about not being perfectly straight. If you can get the lacing out of your belt lay it out flat and measure the length of each side. Belts will stretch or may not have been laced correctly to begin with. Wide belts will often not stretch evenly side to side. Then they are curved when laid out flat. On longer conveyor belts this makes then junk.
You can try measuring the belt in place by wrapping a tap measure around the belt/rollers.
Conveyor belts are frequently blamed for belt tracking problems and in most cases this is unjustified. The failure cause is usually to be found in the installation itself and may be the result of poorly adjusted pulleys and rollers, incorrect application of belt tracking measures or faulty design. It is therefore essential to be fully aware of the basic characteristics of the different belt tracking measures and for these to be employed correctly.
A distinction needs to be made between basic and additional measures for belt tracking. The former are appropriate for maintaining a correctly aligned belt in its central position as long as no great external influences are exerted on the belt, such as transverse forces. The latter are necessary when the basic measures alone are either insufficient or inappropriate to control belt tracking sufficiently.
Regardless which measures are taken, the following conditions are essential for problem-free belt tracking:
What happens if a conveyor has no tracking measure at all?
Where a belt runs over cylindrical pulleys that are at right angles to its directional path, then the forces acting upon it will be parallel to the running direction of the belt. No tracking forces are exerted on the belt.
In fact, the belt is running in a state of unstable equilibrium and would run off immediately if subjected to the slightest external factors such as off-center loading of product, dirt between belt and pulley, belt distortion or lateral feeding or diverting of goods.
The same scenario applies if one or both of the two pulleys are not positioned accurately at right angles to the belt running axis. The belt will inevitably run off towards the less-tensioned side.
The belt tracks to the side with the least tension.