WLAN Password forgot

I use Volumio on Raspberry3 with Audi DAC and touchscreen. It runs very well.
Problem is wlan access:
If i put my password in the Volumio and chose the wlan, it connect well.
After some shuttdowns (regulry) he did not connect. The volumio sees the wlan network but had not connected automatically. I have then put the password again in the field and connect, then it works.
Why does he “forgett” the password sometimes ?? (Sometimes means after every 10 shuttdown).

I make settings only via Touchscreen, or via IP Acces to the GUI from volumio.

Thanks a lot.