planted some tomatoe seeds in our pollytunnel went next day and fond mice have been looking for there acorns,they had left some in our other plants we grow during winter, they left little holes in top of pots
Put them somewhere warm indoors - I use the water heater cupboard - until they germinate. Then keep them indoors for a bit. Mine won't be going out to the mini-greenhouse till we get some warmer weather next month.
If it is mice may some where more secure may help.
Ours have germanated on the window sill at home first. It is south facing and has a radiator under it which does help.
Last year I panted about 50 pots od broad beans in 3 inch pots and left them in my lottie greenhouse.They didn't germinate, eventually I started to look in the pots, no seeds, not one!! I mentioned it to someone else and they told me the mice dig them up and carry them off. I put more in the same pots and took them home in my at home greenhouse. Later on we were tidying up a big pile of pots and stuff in the lottie greenhouse and we found the beans,all lined up like soldiers in neat little rows stashed by the mice, all germinating nicely in the damp corner so I planted those in more pots and took them home too. We had so many beans that year but laughed about. I learned first hand what the mice can do. XX Jeannine