I'm sorry to say its likely its because its contained in a pot - these small trees will reach anything between 6-15 feet, and yours is unable to grow properly because it doesn't have enough root room. If you can find somewhere in the border for it that's very sunny, you may have more success.

This species is a common weed tree here in the American Southwest, and rarely blooms before the trunk is more than 15 cm in diameter.

Hi, welcome to GoY, I agree with the above, but I have to ask, why is the plant so low down in the pot, having it that low, is effectively reducing the size of the pot, with a small pot, you would normally leave about 1/2- 1" from the top of the pot for watering purposes, and even a very large pot, only needs about 2", yours seems to be about halfway doen the pot, so if you can't plant it elsewhere as Bamboo suggests, repot it with more compost, Derek.

The compost also looks very coarse, as if it is actually what we in the States call "mulch", or "planting mix", rather than a proper potting compost.

Thank you all-for welcome & for responses. I am encouraged! Trawling for Internet answers not always rewarding & response from people is so much better!

I had wondered about possible 'pot-constraint'. This pot had housed a 40th wedding anniversary present from children of a large Abelia grandiflora, now transferred to new (scaled-down!) garden at 44yr & counting..! The tamarisk was an aspiration, having had failure at previous heavy clay garden. The base compost is good, only topped with what has been 'composted' from this garden's cuttings etc. I will reconsider siting re garden-this is a very pretty but modestly sized cottage garden...

I lik the habit of the tamarisk where it is-any suggestions for a 'floaty' follow-on for this pot & location..?

Can't see how deep the pot is, but judging by the rim, it's not that shallow. I can think of a couple of perennials, but they won't look good in winter, obviously - Thalictrum varieties (though you'd need to provide support and turn out and divide every couple of years) such as T. delavayi, T. aquilegifolium - there are others. Or Lychnis 'Jenny', but that does not like to dry out too much...