The normal problem is from picking up some debris in the pickup port in the lower unit. There is a vee shape hole in the leading edge just above the bullet housing. Run a paperclip in that hole to see if that's clean. Next is the quick connect fitting in the lower unit flange connection, in front of the shift connection. Raise the outdrive about half way for easy access.. It has a detent button to remove the fitting and takes a little persuasion, twist and pull up, to get the connection off. Look for debris, hose condition etc.