As a first step, turn on Keep alive in Blokada's App settings section. In this case, Blokada's priority will be increased and it can help to stay online; but please note, your system still can effect this behaviour. If you set it, and you still see that Blokada is stopped time after time, check your system’s settings and turn off every battery- and memory-optimisation for Blokada. Unfortunately it is different in every system, therefore please check your phone’s settings or follow the documentation on Don't Kill My App.
Another option is to enable Always-on VPN. To do it, open your system's settings, go to Network & Internet, choose Advanced, there VPN. Choose the gear icon next to Blokada, enable Always-on VPN.
Other source of such behaviour can be that you are using a memory cleaner. Set Blokada on allow- or exclude list. If non of this steps helped in your case, join our Telegram group to debug this misbehaviour.