I have a John Deere 7200 12 row X 30 inch liquid fertilizer Corn planter It has the red ball squeeze pump fertilizer system with single disc fertilizer openers. I cannot get all of the rows to distribute the fertilizer equally.Typically 4 rows work good, four rows are fair and 4 rows put fertilizer down poorly It has for 100 gallon fertilizer tanks. Does anyone have any suggestions on what I should do. I have tried everything and it does not work. It started acting up about two years ago. Before that it worked good.