The thing is about Flexible Syns, they are meant to be used as a tool for special occasions - yes, they absolutely are part of the plan, but if you use them regularly, they will impact on your losses. If you find your losses not going very well, can I suggest that you cut back on the frequency with which you use this tool. If you are using them EVERY weekend, you may well not be actually doing yourself any favours.
This is a post I wrote about Flexisynning a while back now that explains what it really means. Hope that helps.
http://www.minimins.com/slimming-world/186910-flexisynning.html