If you have more than one apple tree, a single pheremone trap won't be enough to solve the problem. Even with a single tree, often the pheremone trap does not save all the apples from harm, but acts as a useful guide for when to spray. It may also be that, despite the recommended time for hanging these being early May, this year we had a very warm spring, so its possible they were busy at the end of April rather than waiting till May.
The other thing to consider is that Apple sawfly, which invades the fruitlets when small and usually causes those to drop, with a clear entry hole in the side, doesn't always have this effect. Sometimes the sawfly enters and dies before reaching the core - so the fruit stays on the tree and matures, but has a brown tunnel in it which stops short of the core.