I've started to use a declined sit-up bench, but when I use it I don't feel a work out in my abs. Also when I go all the way down to lay my back on the bench I feel a pain in my lower back after I do some reps and when I I go half way down I'm working out my upper thighs instead of my abs. I don't think it's because I'm not doing enough reps because when I do sit-ups on the floor I feel my abs working o.ut after only 15 reps

I used sit up benches and they rely on your knee to press force against the pad for leverage taking that workout from your abs.
If you don't even feel the workout though do what I do and just hang from your pullup bar and do situps or jack knife situps it will increase the intensity of the workout.