I'm on the same boat as you are at one point or another, as I live in an area that does not provide mainline flights to anywhere; my recent experiences that prevented me from plane spotting a few CFB charter planes this season are:
-Went to the airport to plane-spot a SY B738 taking off from COU for around 1150 CDT; delayed for an hour and 15 min; had to bail for a CrossFit workout; went home instead of going straight back to the airport, as the plane had taken off 20 minutes after I had finished my workout; would have missed it anyway as the workout was 21 minutes away.
-I have planned to plane-spot the SY B738 (again) departing COU, but I have arrived at my CrossFit workout 15 minutes late, enough for me to get pulled behind by half-an-hour; by the time I had left my workout, the plane had departed already; I was angry at myself for not having left for the workout on-time; as I would have gone for the workout at the top of the 12pm hour.
-I was planning to leave work at 1500CST to plane spot a DL B752 from Vanderbilt and then go straight to a 4pm CrossFit workout, only to get stayed behind to help out with other duties and then wait until a 5pm workout; this complicated my plane-spotting plans, though I did manage to plane-spot the same aircraft yesterday (coincidentally the next day) after my second job at a Mizzou FB game as an usher and went back to my normal job after spotting (hint, it was a late-afternoon flight so there was still daylight out and I was lucky enough to spot it before the early sunset).
While I wished that all goes out my way to plane spot charter aircraft, there are certain situations that would prevent me from doing so. As for the SY B738 photos that I've taken, I was happy to have a couple of them accepted to a.net last year, as well as some DL B752s from the past two seasons, and while I would want to plane spot some of the mainline aircraft that I don't have on my a.net profile, as well as some that I already have, there would be some that I could not either plane-spot or get edited photos of those accepted to the a.net DB. In some situations, there would be other planes out there that I would like to spot that don't fly to my hometown airport, such as the 767, 777, 787 Dreamliner, A330, A380, etc, most of which I don't have photos of on my a.net profile.
End of my long rambling; just wanted to vent my feelings out so that everyone's stories are heard and understood.