My mum never likes the presents my sister and I get her, and it's so disheartening. She tells us why she didn't like the latest gift, probably so we don't waste our money in future, but it's too late when we've already spent it, and we are no wiser afterwards, as she never tells us what she would like, so other than adding to the very long list of things she doesn't want, we are back to square one every birthday and Christmas.
At her age she has everything she needs, and she can buy everything she wants. She doesn't like sweets, doesn't wear accessories or make-up, prefers her own jewellery, doesn't like bath products and gets through one bottle of perfume every few years. Candles or diffusers are a non-starter as she has a cat, and she's not a fan of gadgets. As you can see, that limits our options, yet she expects us to surprise her with something she'll like, and always lets us know that we've missed the mark and she's given away the present because she won't use it, or that we really shouldn't get her another one, as it's not her sort of thing at all.
Honestly, Cambsnan, I wouldn't tell them. They've bought you things out of love, in the hope that you will like them. If you can let them know what you would like, that would be great (I wish my mum would do that), but if that's not how your family operates, I would just pretend that you are delighted.