I agree with Joe. Even if you can provide some screen shots of how your actions are set up that may help. I tend to stay away from the "Use as Filter" actions as they are the most generic and go from source sheet to ALL OTHER SHEETS on the dashboard and for ALL FIELDS. These create the "(generated actions)". I would surgically create the dashboard actions in the actions dialog box and control each one with the "from" and "to" as well as perhaps target specific fields rather than use all fields. It is very easy for multiple actions to get tangled up and conflict with each other.
Does action B work by itself? In situations when the second action filter selects data that is not a subset of the first action, the second action filter may cancel the first. It depends if the fields that are affected by the first filter are present in the report that needs to be filtered. The problem that I observed is that both action filters stay selected while in fact not both are applied.
This just happened to me too and I was also dumbfounded. I have one worksheet serving as a filter to three others, each with a separate dashboard filter action. Two of the three were working just fine, with the Action pill showing on the filter shelf of both worksheets and the target worksheets showing on the dashboard when I made a selection from the source worksheet. But the third action was MIA. I even deleted that action and rebuilt it from scratch with the same result. I confirmed that the source sheet was the only source on all three actions, and the target worksheet the only target. The only way I got it to work was by filtering all fields for the MIA action rather than selected fields.
Were you ever able to resolve your issue?