The service engineer may have checked the pads but was there any comments on the report at all relating to the hitch?
If it was just ticked as satisfactory then I would also suggest that you are not compressing the handle right down to clamp the pads onto the towball. However, if the handle does go right the way down with no resistance then I would suggest that you get the service engineer to return and have a second look.
Is it contacting handbrake lever when hitch is compressed? This can occur when hitch has been fitted at later date to caravan not intended for that hitch.