I doubt it's a humidity problem, although clean, dry air is very important, a dryer is always a good idea.
The T45 torch starts its pilot arc by shorting the electrode to the nozzle and running a current through it. The air then turns on and causes the electrode to pull away from the nozzle, creating an arc between the two. With 15m leads it's takes a little while to build enough pressure in the line to "blow back" the electrode starting your arc.
There is an arc transfer signal (XFER) from the powermax you can connect to your CNC controller, if possible. It tells the controller when the pilot arc has hit the work piece and has started cutting, so your pierce time will be accurate.