I have a similar issue with one of my M10 tablets. When I reboot it on restart it does not seem to be able to pick up the Wi-Fi. Everything else I have connects easy to it, but for some reason not this one tablet. The other Ubuntu Touch tablet that I have connects all OK. How strange, it is affecting just one of them.
The only thing I can do is go into Wi-Fi settings, like I am going to add it again. Then before I have chance to add it, it shows as connected in the Settings list. Then it will appear in the notifications bar. I tried waiting for it to connect, thinking that it was taking a long time to auto connect, but this did not work. Only going into settings connects it for me.