As others have said, you may have the three way valve just ahead of the pump set to draw from an antifreeze bottle instead of from your fresh water tank.
Assuming you do have it set properly, here is another tip that may help - I used to have to do this with my last RV that would have difficulty priming the pump from the fresh water tank in Spring.
If it wouldn't prime, I would draw water from a one gallon container using the antifreeze input hose, and while pumping, quickly flip the valve over to draw from the fresh water tank. It always seemed to work for me.
Just a suggestion!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.