I seem to remember that I had the same problem!
The problem is that the FULL reference isn't immediately obvious.
And you don't necessarily enter the FULL reference?!
This is from TNA website:
Record copying
Catalogue references
Our computer systems are very strict about the way document references are written.
Invalid references:
First check the catalogue again to make sure that you entered the correct reference.
The catalogue reference numbers may be made up of up to three letter/number combinations, for example: HO 144/403/B23013.
Sometimes, if you enter a three-part code, you will get an error message.
If this happens, try removing the last part of the reference and put this part of the reference in the text box where you give us details of what you are looking for.
Example: The reference C 14/1186/T37 will give you an error. Instead, put C 14/1186 into the document reference box, and put the last part of the reference into the text box along with any other instructions or information you want to give us. For example: Toplis v Hurrell. Bill, certificate, five answers, replication, nine depositions - part T37
I think for my divorce papers I entered J77/12345 as the reference, and then 'part 310' or 'page 310' in the text box, along with the names of both parties.
I wasn't helped by the fact the my grandfather's surname was spelled incorrectly! (Garrard instead of Garrad)