I've always had difficulty with this, I find it similar to driving - too distracting to allow the necessary steady coordination of muscles. I usually stop walking, stand with my legs slightly apart, then once the flow starts I can sometimes set off again although that will often stop the flow. Fear of leaks is definitely the #1 inihibitor so you must take time to practise and prepare, and test your padding, until you are very confident in its performance. But there seems to be more to it than that, as I have no trouble wetting in front of other people when standing still. I can go anywhere including in checkout lines, parties, in a crowded bar or a 1-to-1 situation, but when walking it becomes way harder to focus on the right muscles to make it happen even in favourable conditions.
The only time it's really effortless is after I've done a poo (which I can do easily while walking), I sometimes flood the nappy as part of the process of relaxing once my bottom is empty, even while still walking. Also my DL GF never had any dfficulty, though girls do seem to wet more freely overall, not least due to straighter plumbing.