we had Dickey johns for many years, made life easier when we replaced them with ravens.       so things you can do, take some fiberglass insulation and loosely fill the center of the cooler with it, it helps more than you can imagine, also clean your strainers, if there are any.     the flow sensor is notoriously sensitive to lightning.  so if it worked okay last year, and now it doesn't, and you had the unit parked out side for the season, and there was some lightning storms in that time frame, could be a real possibility of it being bad.   next take the servo cover loose and see if any water runs out, here again they were notorious for filling with water and it wouldn't drain out,   looking at it it doesn't seem possible,   trust me it's possible.           spray some wd-40 on the servo mechanism to dry and lubricate.
 
 beings it's a dickey john system, i'm going to assume it's a older install, so check breakaways, make sure they are opening fully, check the hoses make sure they aren't delaminating on the inside.