Based on your description of what you did, I'm inclined to think you shouldn't have has the 'successful' pop up.

When importing transactions, Sage does it in two passes: the first pass is to check that all the transactions are valid - and only if that is so does it perform the second pass, which actually imports the transactions. All it takes, therefore is one invalid transaction, and none will be imported.

One of the things that can cause Sage not to perform the import - and what I suspect may be the case here - is invoices to customers that Sage doesn't have records of in the sales ledger. I.e. new customers created in the software used for the invoicing.

What you would need to do is create accounts for these in Sage first (or export them from the other program, and import them into Sage) before importing the invoices themselves.

However, the flaw with this is that you say you've had the message to say the import was successful. What you should have got, if my guess is correct, is a dialogue telling you it failed, and telling you which transactins (line numbers in the csv file) caused the problem, and why. That way you can load the csv file into a spreadsheet and identify the problem (for example find out what customer accounts you need if the errors were all along the lines of 'customer acccount doesn't exist).

If it's definitely telling you the import was successful when it wasn't, then I'm afraid that's one for Sage support.