When your training plan does not seem to work anymore. When a day or two after your training you do not feel any more aches, here is a little trick to help you:
In the same set, alternate slow and fast repetitions.
For example, on the bench press, if you target 8 repetitions, I propose the following strategy:
2 slow repetitions (5 seconds down, 5 seconds up); 2 fast repeats (2 seconds down, 2 seconds up); 2 slow repetitions; 2 quick repetitions.
Good gym!