My titan 400I won't prime what can be wrong with it?

OK, Just solved this problem with my Titan 400 not priming....After searching the internet I got the clue that it was the inlet ball valve being stuck. Most said it was from leaving paint in the system or poor cleaning habits. I thought "Not me, I always do a good job cleaning the sprayer and leaving pump preserver in the system when storing". But I was wrong. After disassembling the inlet valve housing (shown in the manual) I found the valve ball stuck to the seat. After freeing the ball there was tiny bit of paint on the seat but it was enough to prevent it from functioning. Reassembled and it works fine. So maybe just luck of the draw. I don't use the system too often and it had been 6 months since the last use...still I stored it with line full of pump preservative. Symptoms were that when in prime it would not **** from the siphon tube (supply) but motor ran fine. A bit painful fix but not terrible, exercise cost me an hour, but next time would take maybe 15 minutes to correct. Hope this helps.