One possible issue with your formula could be that the body text contains spaces or special characters that need to be URL encoded.
You can try replacing spaces with %20 and other special characters with their URL encoded equivalents.
For example, if the text in cell B3 is “Thank you for your support”, you can try replacing it with “Thank%20you%20for%20your%20support” in the formula.
So the formula would look like this:
=HYPERLINK("mailto:" & B1 & "?subject=" & B2 & "&body=Thank%20you%20for%20your%20support", "send email")
I hope this helps!