Dust and dirt would get over the sensors. They need to be clean. Tell me more if you know the age of your planter. what type of openers do you have? There were all kind of changes in those planters. I have one in the barn, have not used it in 20 years. The only reason I kept it I think it is a prototype unit. As far as the air pressure, there may be a problem with air leaking past the plates. Does your planter have the crome wear strips? Send me a phone number. There were also problems as with any wiring harness of poor conections. We received a planter manual one year in service school. I will look for it this week.