Could not detect any mucin production even in the positive...

Dear Tracey Brown,
Unfortunately, you give very little information, so I can only make assumptions about your problem. I assume you used the same cell lines and materials as described in the protocol. Even if your cells do not secrete mucus or the media change was not careful enough and mucus was aspirated, you should at least be able to detect intracellular mucus.
First, check whether your cells produce mucins qualitatively with PAS/alcian blue staining. If you get a positive result, then the problem is probably the sample preparation for the ELISA or the ELISA procedure itself. Here, you could try to use more cells for the ELISA because your mucin production may be below the ELISA detection limit. Also, the sonification step may not have worked correctly? It would also be interesting to know if you have obtained a proper standard curve for the ELISA for the standard provided by the manufacturer?
I hope I was able to help you and if you have any further questions, please do not hesitate to contact me by e-mail.