Every time I change a filament (PLA) from one color to another, the material does not advance out of the nozzle. After flushing the nozzle and clearly seeing through the hole, I continue the process of "changing material" again. The filament usually does not extrude when the "wait" message is displayed. I repeat the process of "changing material" several times. Sometimes it advances and material does extrude and then I advance onto "printing" an object. Sometimes it does not and I continue to "print" anyway and try to "tune" the "flow". This usually does not work.Has anyone had a problem like this? Any suggestions as to what I should do? Many thanks!