Help with troubleshooting Delta

My Delta flashlight is not turning on. It will occasionally turn on and let me go through the modes once the. Cuts out. I've tried cleaning the threads and tailcap and no such luck. It's not the batteries as they've been charged and work in my other lights. Any ideas on what could be causing the problem?

Did make sure the switch is tightened? If that does fix it then msg customer service.

Did You get it fixed?
The Delta's ICARUS driver requires a HD 5C 18350 cell delivering 3.4A
on maximum setting to function properly as designed.
Will the light stay on in Low or Medium, or does it flicker?
Have You tried to verify the switch is fully tightened
into the tailcap? All that is needed is a pair of needle nose pilers
to remove the switch and then clean the switch's contact surfaces
and reassemble. If this does not correct it, either the cell is not
providing enough juice, or the Delta needs warranty repair.

Also, change the cell by unscrewing the head, not the tail-switch,
as per Jason's recommendation for the Alpha and Delta lights.