It will be a blockage in the Y piece. When you turn the water off to look for it, it will go back down slightly into the pole hose. The reason it switches sides is that it will switch from the left hand outlet to the right hand outlet. The only way to cure it is to completely disconnect the Y piece and blow it back out the inlet. Also, turn the pole hose on with no Y piece connected to flush out the top section of pole hose. Often these blockages are translucent bits of swarf from the tank manufacture which is why they are very hard to see as when they are surrounded by water in the hose, they look much the same as the water.