If it was working well, and now it's not, something changed. If it's not the bands (new supplier, etc.) then look for what else may have changed. Consider that something may have broken or failed.
Examine the tracking plate closely on the idler wheel... you may find that it has weakened, or one of the hinges has broken. Perhaps it was bumped by a machine and knocked out of true by a log.
Check for loose or perhaps broken bolts that fasten the tracking plate to the frame. Also check that the idler wheel bearing is spinning freely. These are often "sealed" bearings and they have a lifetime. After a couple 1000 hours you'll need to replace it.
On the drive side, check the main bearings and ensure they are rotating freely, the shaft is true and not bent. Check the grease fittings (usually dashpots) and make sure they are pushing grease into the bearings.
After all that, check your bandwheels to ensure they are co-planer. Lay a straight metal level or ruler horizontally across the face of the bandwheels about 1/3 of the way down from the top, and above the axle. Look closely at the 4 outer edges of the bandwheel that touch the straight edge, and check for any gaps between them. Do this again below the axle 1/3 of the way, and check again.
If you find any gaps, you should adjust the bearings or the tracking plate to get them dialed in. If bandwheels are not perfectly co-planer, the band stretches unevenly from front to back as it rides around the wheels and those stresses will cause metal fatigue more quickly than normal. Sometimes you can look at the blades and see very slight cracks in the other gullets of a failed blade, or on the backside of the blade working its way toward the gullet. If you see this in your broken blades, it's a sign that the bandwheels are not co-planer and need to be adjusted.
Also check your roller guide bearings and make sure they are not seized. (You'd probably have noticed that as they tend to scream when they won't turn.)
Post a picture of your broken edges of the blade and someone can probably tell whether it broke because of stress fracture or from fatigue. That will help narrow it down at least.
Best of luck to you.