Doser 2.1 losing prime

Hi I’m having a problem with my fourth channel losing prime. I can prime the pump and calibrate it ok but within a day or two the line is dry again. I’m using same tubing and container on all 4 channels and only have the issue with one. It’s the first time I’ve ran this head since buying the doser as I previously only required 3 heads so I doubt I’ve worn anything out. Any ideas on the best way to trouble shoot this issue? Thanks