I agree that the amount of tubing stinking out past the clamp is critical. However, I have also found that different runs of tubing can have slightly smaller inner diameters and be very tight. In these cases, I have found that a little water on the T can help lubricate along with warmer temps
2018: 684 taps: 525-3/16" Gr.,159-3/16" Hybrid; Mountain Maple Super Sap Sucker, CDL 600 RO - 131 gal.; retired from WVU in May
2017: 439 Taps - 3/16" Gravity, Goulds 18GBS15, CDL 2X6 Drop Flue, Wes Fab 7" SB Filter Press, Smokey Lake WJ Bottler - 90 gal.
2016: 258 Taps - 3/16" Gravity, Honda WX15, NGMP 2X6 Flat Pan, Deer Run 125 RO - 68 gal.
2015: 20 Taps - Jugs, Turkey Fryer and 2-Burner Stove - 2.5 gal.