It's possible to get leaves or insulation or other crud in the evaporator housing which clog the top of the drain tube opening. You push them up, water comes out, you withdraw wire, the drain clogs back up. Eventually they may rot and flush out. Only way to be sure is to remove the evaporator housing and clean it out. The other possible thing to do is fashion a sort of filter screen that will stand proud of the drain opening inside thereby keeping it open. You can also put a vacuum on the drain to try to suck stuff out.