help in troubleshooting 3/16 line for next season

[GV2]

I had a 3/16 line this year with 21 taps. The sap in the line at the top 7 taps was suspended. I replace the tee where bubble flow started downward, but that did not get the upward sap moving. A few days later for the heck of it, I pulled the next tap up for a few seconds and everything began to run, even after I put that tap back in. a few days later, sap was suspended above the same location. It was nearing the end so I just let it be.

Does anybody know what may be the issue? Two possibilities I came up with is perhaps poor slope in this section of the line (though the sap did run down when vacuum was interrupted), or too many taps (overloaded). My plan would be to put fewer taps with more slope, but additional thoughts about what was possibly going on is appreciated.