If they wiggle around the liner moves with you
Top tip: a hot meal shortly before bed time helps an awful lot.
Synthetic and down insulation work the same way in that you heat it then it keeps you warm, so if your body is busy digesting food it’s generating warmth.
Another thing to remember is that your sleeping mat can be critical. You’ll lose downwards too. A decent mat will stop you losing heat to the ground. Opinions vary on the best way to do this but again good mats are not cheap but worth it if you get a good night’s sleep