Answer: There are several possible reasons. The most common reason is that the foliage is being removed too soon. It needs to be left growing and then allowed to dry up and turn brown before you remove it. That is how the bulbs replenishes its strength to bloom the following year.
Another reason can be lack of nutrients. You could try giving them some complete granular fertilizer such as 10-10-10 in the early fall (when the bulbs begin to root) and again in very early spring just as they begin to grow for the season.
Daffodils and the small species tulips usually bloom well for many years in one spot, but can eventually become crowded and need to be lifted, separated and replanted further apart.
Some of the larger tulips will also respond to this treatment. However, many tulips are just not terribly long lived in the garden and need to be replaced after a couple of years at the most. I hope this helps.
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