They only flower in the early spring, somewhat similar to stone fruits. If you prune them hard in the late spring, they will sometimes push flowers on the new growth... but not always. If your quince have not flowered this year, I don't believe that any fertilizer will get them to flower now - this late in the season. Rather, just make sure you leave your tree alone (no pruning) from now until next spring. People sometimes mistakenly prune late in fall (or even in winter) and unknowingly prune off all the flower buds!