Rows in Calc not sorting accurately. First filter working and second mostly ok, but one instance where W is placed before S

Rows in Calc not sorting accurately. First filter (Surname column) working and second (First name) mostly ok, but one instance where a row with W in First name column is placed before the row (same surname) with S in First name column. I have updated my version of LibreOffice (just now) but the fault persists.

Oh but look at cell … hmm… oh, wow! I just realized I cannot tell you where the problem is, just because you didn’t provide the document!

Sorry, Mike, I am not very experienced with this forum. Is there the facility to upload a file? I could do so (if there were a way to) but it’s the whole of my address book, with about 300 entries in it. (The problem occurs at rows 72 and 73, which are the phone numbers of the two locations of our doctor’s surgeries.)

Please create a small document, with e.g. 3 entries, that are the entry put wrong, and two its neighbours. Remove all data that isn’t relevant, like addresses, which doesn’t take part in sorting. Check that when you try sorting this small document, you get wrong results, and then put the small document to a share on Internet (like Google Docs, or DropBox). Put a link here.

Also when you edit your question, there’s a clip icon, allowing you to attach the document here.

Perhaps the most common cause of sorting errors (and also mismatch errors) is leading and trailing spaces. Is there a leading space before the “W” you mention, or a trailing space in the surname belonging to the “W person”?

Thank you, Mike and Keme,
Somehow, between you, you have sorted my problem out.
There isn’t (as far as I can see) a means of Showing Non-Printing Characters in Calc (as there is in Writer) but I kept fiddling about trying to eliminate possible redundant spaces. I didn’t appear to be achieving much and so I began to copy just the First Name and Surname cells of about seven rows including the rogue pair, made a new spreadsheet and pasted the little block in. The W and S entries sorted themselves properly. I then went back to the original and saw that the selected block that I’d copied was also right now!
Thank you for some useful tips I will bear in mind in the future.