What's there form?
If form is good, goals will come!
If not, the following is what i'd do;
1. Try switching there position. Ex. Try your left striker in either the middle or right slot.
2. Play them in a different formation. Some players play better in a 4-4-2 classic than 4-5-1 V-style and vice versa. There is also the little thing of counter-formations too.
3. If their form remains poor; get rid of them. Some player will perform poorly regardless of there quality (# of stars) and there is little that can be done to fix it.
EDIT: What formations have the opponents that have given you trouble been using? If that formation counters yours, that could explain why the goals have stopped also.