A client recently upgrading from Standard 12.2 to Classic 15 HF3 had some single sided transfers that needed cleaned up in ProTrust. They were in pretty good shape with 110 of them in a 10 year database. As they were cleaning up the error message Unable to match the selected transactions. Ledger locking failed started appearing.
The first thing I did was clear the PTLocks folder, that didn't seem to help. I found the problem by executing a search of Firm File of " *", a space followed by an asterisk. I expected that to return only files that started with a space, but got to wait for 37,000 records to load. Use the Go button for faster responses, as the List of Ledgers takes longer to load tons of records, and might not like to close.. The first 14 records were defective, starting with a space or other incorrect character. There were also a few files at the end of the record list, and a smattering of 3 file ledgers renamed and showing up in the group of transfer ledgers.
This sometimes can be seen in the Single Sided Transfer utility. In the top highlighted line of the image, the File Number column is indented with a space. Not all files with issues will be visible this way.
Of course it's not a bad idea to verify if ProForm has the same problem, some ledgers might end in PFD, and are likely accurate with ProForm. Others with errant starting characters, extra trailing characters, or entirely renamed are quite possibly inaccurate and no longer match what ProForm has for the file, these need fixed before the Single Sided Transaction program can do it's job.