Shaky, noisy and hard to open bi-fold doors
Bi-fold doors are just terrible. Even at their best they're often jiggly and awkward. They're such a pain that half of them end up perpetually open. 

I'd look at replacing the hardware with a kit from your local store. They'll include track, door lap guides, upper rollers, and bottom pins and brackets. 

Another problem is when the carpenter puts the knobs in the wrong place. They're often set right near the hinges. This makes the closing force all wrong, which results in premature wear and deformation of hardware. Ideally, the knobs will be in the center of the inner panel. This allows force to be applied in all necessary directions for both opening and closing action.