After 5 years in the same location it may well be that the soil is exhausted. Once the leaves have died back naturally, try spreading a good layer (6 inches) of organic matter (farmyard manure) and lightly fork into the soil. Repeat the process in late autumn and you should have a good show of flowers next year.
If you don’t care for the mess and stink of good old fashioned ‘Muck’, you can buy a bagged version at the supermarket or garden center that has been dried to remove the smell and make it cleaner to handle. Of course this will be more expensive than collecting a trailer load from a farm or stables.
If you don’t care for the appearance of the leaves dying back, then you can always lift the bulbs once the leaves start to turn yellow. Gently rub off excess soil and store the bulbs in a single layer on 2 or 3 layers of newspaper in a cardboard or wooden box in a dry place. Lifting the bulbs gives a good opportunity to dig in some manure, as does digging the ground over prior to replanting the bulbs in autumn. Clumps of bulbs should be teased apart at replanting time. Daffs like to be fairly deep in the soil, so the top of the bulb should be a good 6 inches below soil level.
Invariably, when a plant has been in the same place for a number of years, and then in the absence of obvious disease or attack starts failing to flower or grow properly, the answer lies in the soil. Top dressing with a good quality granular fertiliser in late autumn, and again in early spring, will keep the soil well nourished and able to sustain the desired growth and flowering of your plants.