Bronica ETRSi Only 12 Images

[MarkH]

Can anyone help please, I have the Bronica ETRSi with the mkII winder. I'm new to MF photography and I've only recently got this camera and so far I have only put 2 rolls of Fujifilm Pro 160ns through it. My problem is the winder seems to wind on to the end of the film after the 12th shot. As it's a 645 camera I was expecting about 14-15 shots I have another back which has Ilford Delta 100 loaded on it, but I haven't finished this roll yet so I don't know if it's a film, back or wider problem. All the different elements such as winder and back are genuine Bronica ETRSi and not third party in any way, that is if you can get third party composites. Has anyone ever experienced this before please? And if they have is it fixable without risking making things worse? I'd rather lose 2 or 3 images a roll than damage anything further. Thank you in advance for any advice.