as was suggested the drive tire may be worn down
there are two ways to address this, either replace the tire or you can reshim the casters to compensate for the tire wear if it is not completely worn down to the wear indicator on the tire.
in most cases replacing the tire will fix this.
One other problem i've encountered with these type lifts is if the floor is uneven, some concrete floors even though smoothed down flat the still have low spots in the floor and when you run over these spots the rigid frame/outrigger/caster setup doesn't flex enough so the tire may loose traction in certain spots. This is where adjusting the caster shims is done to compensate for some situations but this only gets you so much adjustment. If you remove too many shims it may apply too much force on the tire making it hard to steer the unit so it is usually a trial and error adjustment. Also keep in mind if the drive tire is replaced the caster shims will have to be readjusted back. I checked this unit manual on bigjoe support and it does not show adjustable casters other than to add or remove shims.
In the end the simplest solution would to be just to replace the drive tire.
The part number for this tire is 1120-200001-10 from Big Joe.