It's not shooting

In auto mode it can take some time untill the Big Bertha begins to shoot. Especially when the enemy is far away. The artillery seems to “scan” in a growing clockwise circle. When it recognized the enemy, it will begin to shoot. In the long term it should be really effective against nests.

It has no power / disabled by script

When you hover with your mouse over the bottom of the Big Bertha you can see the Big Bertha power connector. This red rectangle needs to be powered. By default, it uses 10MW while shooting.

How to change mode

There is a menu that pops up on the top left of the screen when you click the Big Bertha.  Depending on how many mods you have installed, it is shifted to the center. Because of that, it  may be that the menu is behind your inventory.
In that case, close your inventory with the X Button or move your inventory a bit down.
The Big Bertha  window is closed with “E” or the button “Close”.