Am I expecting too much?

Teenagers can be very thoughtless. I understand you feeling that she should have said thank you.
It's hard to know whether to let it go or mention to her parents that you were wondering what she had bought with the money as you hadn't heard from her.

Yes definitely thoughtless, a simple text / e.mail would have done, I bet her parents have asked her to message you but she has "forgotten" I would make enquires as AGA4 as suggested. Perhaps a little conversation about manners at a later date, saying how things have changed over years might prove as a little reminder.

No, you shouldn't contact anyone regarding this, imo. A true gift is given unreservedly and, if you are as unhappy about this as you seem to be, then give absolutely nothing in future.

Perhaps you could ask what she intends to do with the money at some point. Should it be earmarked for a car, ask what sort of make/model she’s looking for.
I sent a nephew a cheque on 2 occasions without thanks - the first time I mentioned I’d seen he’d banked it safely and the second time I reminded him he hadn’t!

I would say

"granddaughter, I just wanted to check you go the money for your birthday ok?“

Give her a chamce before you write her off as rude or ungrateful

Saves an argument with her or parents and spares her guilt if she forgot to say thank you.

1 vote for VS

My granddaughter has never said thank you, unless I have handed her her present on her actual birthday. So if I have posted it, or left it a day or two early, I hear nothing. This year she will be 21 so it will be the last time I give a gift.

She is eighteen. Presumably you didn't see her to give her the money personally, so I would ring her up/ email/text and simply ask if she has received it. Don't involve her parents.

Birthday money !!! Don’t even go there ! 3 children, 2 daughters-in-law and 6 grandchildren. I will die poor, but happy ?

I would not speak to the parents, your granddaughter is an adult now, I would ask her what she had planned for the money, this would probably remind her to thank you. However its really to late now, she should have thanked you at the time.

I agree about not mentioning this to her parents. Ask her what she's done with the money and hopefully this will elicit a thank you which she should have done when the gift was received.

I have three nieces and a nephew. Two nieces and the nephew always send thank you messages. Not the other niece.

Young adult grandchildren don’t seem to be in the habit of thanking unless the gift is put into their hand by their gran/aunt/etc. I’d try to accept it isn’t personal and if yiu feel strongly, not bother sending gifts in future

Sorry I disagree with most posters. I gather this was a larger amount than would usually be given as it was an 18th birthday gift.
I would certainly be asking my granddaughter if the gift was safely received. I really don't think we should excuse bad manners.

I don’t think we should excuse bad manners either sodapop. It seems that bad manners are regarded as acceptable by some, but not me.

Definitely don't speak to her parents. She is 18 and responsible for her own behaviour/forgetfulness or whatever

Why not just do as VioletSky suggests...

Quite agree, if someone gives me something I really want then undoubtedly I would show my gratitude, I gave my God daughter a very nice present when she was 18 and the thank you note came straight back expressing her delight and gratitude for a large cheque to help her through her first term at Uni.