Give alerts to why effects can’t past into Source tab (not Source Monitor) in Effect Controls.

It would help if there was a warning prompt alerting the user that an effect can't be pasted to the Source tab (not the Source Monitor) from the master clip tab, in the Effect Controls panel, because of whatever is preventing it from being pasted. 

In this case it was a mask on the Mosaic video effect. Once the mask was removed, effect pasting was avialable again into the Source tab. 

This was all on an adjustment layer, by the way. But I'm sure it applies to other video/still clips as well.