If the clumps of daffodils scattered in your established borders have healthy leaves but give very few flowers then most like cause is that they have become overcrowded. You just need to lift them, separate them gently – and replant them farther apart.
The time to lift the clumps is after the daffodils have finished flowering and the leaves are turning yellow. The leaves are useful in that they remind you of the exact whereabouts of the bulbs. Unfortunately this is not always the best season to replant because the soil can be too dry: so keep the bulbs in a cool dry place until autumn rain begins to moisten the ground in about September. Sort the daffodils bulbs out before storing them. Throw away any soft or damaged ones and rub off excess soil from those you are keeping, then spread them out on a tray. As the summer progresses, remove any leaves, roots and skins that become dry and brittle.
If you want to plant the daffodils bulbs back in the same positions in your borders, prepare the soil thoroughly first. This is important, for when daffodils are grown in the same soil for many years, it becomes very impoverished. Dig in some well-rotted compost or manure and add a generous dressing of bone meal.
When you put the bulbs back in the border, set them in holes at least three times the depth of the bulb. A bulb 2in deep, for instance, should go into a hole at least 6in deep – deeper if you hoe the ground at all, to protect the bulb from disturbance – and should be covered with at least 4in of soil. Set the bulbs 4-8in apart if they’re the large varieties. 2-3in apart if they’re small. You should get a refreshed display full of flowers the spring after next.