You know the simple answer, stick with a program until it stops working. But don't be afraid to play around with a program to make it fit your needs.
I personally suck at benching. Low reps work great for me on my dl and squat, but do not help my bench much. So when I pick a program, I usually have to find a way to get more volume in for bench. I do this one of two ways, more reps or more sets.
Say its a linear progression and week one has you doing an 8rm on all three lifts, I usually make bench a 12rm or 10rm. Or if it calls for 3 sets of 8 reps, I add in another set for bench.
This is very general, but the point is, its okay to tweak a program to fit your needs as long as you know what they are, and you implement the changes intelligently.