There are several reasons why the perennial or the annual plant coreopsis will not flower. The seedlings need warm soil and full sun. They also need a soil that is not really rich in nitrogen or else they may produce a lot of leaves but no blooms. They do not like to be overwatered. It may be easiest to solve this mystery if you send a photo or describe the different conditions that have made the two plants produce flowers. Do these two get more sun? Are they on higher ground so they get better drainage? Perhaps a high nitrogen lawn food has moved into the roots of the non-bloomers and the other two plants avoided all that nitrogen. Coreopsis are easy to grow plants as long as their demands for sun, well drained soil and warm weather are met. All will bloom longer if the spent or faded blooms are removed to encourage more buds. Did you grow the plants from seed? Sometimes saved seed is from inferior plants or the seedlings need a longer, warm growing season than what your climate can offer. Growing coreopsis from plants already in bloom from a nursery is an easier way to get sure fire flowers for the summer.  Keep growing, Marianne Binetti