I'm not sure 'cause i just started making missions myself a couple of days ago to test some scope views and torps, but I noticed my test missions stopping before I could even get them started - I fixed that by making the objective something that couldn't be completed (in my case - I set it for sinking more tonnage than was present in the mission). You might want to try that (either add a second objective or make the first one un-achievable - the latter approach would mean you wouldn't get the satisfaction of completing the mission, but you'd be able to keep blasting away anyway).
Good luck!
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