Hunger! Basically it takes a lot of effort for a plant to produce flowers. They need a lot of food to do it. So unless you have refreshed and replanted every year then the problem is obvious. Feed them.
The other thing with tulips is that they need a good dry baking over summer to ripen the bulbs.

I agree with Owd, also in pots they may have got too wet over winter. But feeding is important now. Check that the bulbs and if still firm then they will recover, but if not then I'd probably buy fresh ones.

I often plant them out in the garden in their 2nd year.

Not sure what has happened as I feed them with my other plants throughout the Summer.

I read somewhere that it could be that they were not planted deep enough but this is not the case as they have actually flowered for a couple of years.

I will continue to feed them this year and if nothing comes of them next year, then will start again.

Last year I removed mine from the containers after feeding them and popped them in another one with no compost until I could deal with them. But I wasn't well and didn't. So in the autumn I planted them anyway and they have been beautiful. Must have enjoyed the sun on the bulbs. But I had been careful to choose varieties that were listed as coming again. Sarah Ravens catalogue tells you which ones will.

After that length of time the soil itself will be exhausted, so it would be better to empty it out and start again with fresh compost. Feeding the bulbs is not the same as feeding the soil.

In November I will empty the pots and re-pot the bulbs.

Can I use the same bulbs or would you suggest getting some more.

I have lived in the same house since I was born and we have crocus and Lily of the Valley from when I was little and they come up every year without fail (and no, I will not give my age LOL). We also have a white lilac tree which does well every year from when my parents bought our house in 1963.

There is a big difference between the soil out in the garden and the compost in a tub. In the garden, bacteria and other soil organisms have easy access to refresh the soil, whereas in a tub that access is not possible.
As to buying new bulbs, that depends on the condition of the ones you take out. If they are a decent size and solid, then by all means re-use them.