samsung not sending text message to one contact only

[HLB]

Hi.  My Samsung (S20 ultra) has stopped sending messages to one contact.  I have called and spoke to person so know the number is not an issue or I'm blocked.  Its just the text message.

I have reset the phone, switched on and off. 

Any other suggestions please? 

Thanks 

[HLB]

what did you do to fix it?

[AshtabS9Ultra]

just went into message settings and turned chat settings off, or if you kind find it that way go into connection setting and turn off rich communications. I have had endless problems with it but all working soundly

[Pugs1957]

Do you get any error message?

[HLB]

nothing at all. I switched on delivery receipts and with other messages I get an instant receipt. with this number, nothing. like I said I have rung the number and spoke to my sister but nothing has gone through to her

[HLB]

tried everything - still not working
have now spoken to Vodafone who said they have changed some settings and to give it 2-24 hours

[HLB]

Well I am af a complete loss. I have been on to my phone provider and my sister hers. both say there is not an issue. we have both done a factory reset (which was fun), I installed a different messaging app. nothing has worked. I can still ring her and that's fine, can receive text messages from her, I just can't text her
any other ideas please- this is driving me crazy now