Changing the start gcode won't help for the UM2, in its default mode as the start/end gcode is implemented on the printer itself, whereas its configurable for the UM1.
However, as Anthony says, plastic can tend to ooze from the head if it's sat with the nozzle hot, so that when the print starts, it doesn't always extrude at first, until the nozzle has refilled with plastic.
Have you updated to the latest firmware? (As shipped with last week's release of Cura 13.12?) There's a bug in that that causes excessive retraction at the filament after you abort a print. Could that be the problem? For now the preheat and manually advance option is the best fix, but hopefully they will have a fix for that very soon.
Don't give up hope! These are new machines, and there are still some teething problems. But UM is working hard to fix them, and there are a lot of folks on here who can help with any issues you run into. The results from the printer can be stunning, so don't get downhearted - you picked a great printer, and it will give you some great results.
Sometimes the PLA drains out if the nozzle is hot but not printing (like from the end of your last print).
I found that making a simple change to the start.gcode (found from Cura in the Start/End-GCode tab) to make more filament extrude in the beginning, here is the new portion of code (commented old line, newline added below):
;G1 F200 E3 ;extrude 3mm of feed stock
G1 E14 F300 ;14mm at 300mm/s
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