I think I know the answer to this at least in part. I've lost a total of 9st previously with SW (lost 5, gained 4, lost 4) but never got to my target weight or anywhere near (about 2-3st off it at best).
Both times I was eating awful stuff I hated...yoghurts, rice packets, those pasta & sauce things..plus the hi fi bars etc. I also never had any syns, or no more than 5 a day.
Most days I didn't like what I was eating. It felt like a diet - not being hungry but eating horrible food I didn't like!
My plan this time is to try and eat stuff I like within reason. I think this means I'll need to use my syns more.
Any other tips that those who have lost and kept it off can share please?