Flowering Quince (Chaenomeles), like many early-blooming shrubs, form flower buds on growth produced the previous year. On young plants, flowering on more interior wood, as it is more mature growth, is normal. If any branch tips were pruned (or eaten, if deer were around) between last summer and this spring, the flower buds on that growth would have been removed and not replaced by the plant. If no pruning occurred, then perhaps they are simply flowering to the best of their potential given their age and conditions they are growing in. Quince tend to flower best in full sun - at least 6 hours a day in summer; this site appears to be partially shaded, but we realize that may simply be long shadows if the pictures were taken in the morning or evening.
The health of these plants appears to be good, so there is little we would recommend doing at this point. Bloom-boosting fertilizers can only increase flowering for a plant up to its potential, as it is still limited by other factors such as light availability, growth rate, and other potential environmental stresses throughout the growing season that influence the formation of buds. The nutrients that are the focus of such fertilizers are not often deficient in our soils. If you prefer to shape the plants through pruning (an aesthetic choice, not necessary to the plant's health), then trim them later this spring once flowering has finished. On older plants, removal of entire stems that have aged and declined in flowering can be of help (called "renewal pruning"), but these plants look young enough that this should not be needed for at least several more years.