Cmd+click allows you to select a frame that is under another frame—so that pinpoints the issue. Cmd+click to select it again then Object > Arrange > Bring to Front. Deselect, and see if you can edit the footer directly.
If that takes care of the problem, the next focal point should be on what was overlapping the footer. The next step I would take is the go back to the A-Master and select Edit > Select All with the Selection tool (black arrow) active. This will select all the frames on the page, and you will be able to see where the problem frame is, and take it from there.
Please come back and let us know if you need additional clarification. Sharing the same same screen shot with all the frames selected and the Footers layer opened in the Layers panel (click the white arrow in front of the name Footers) would be helpful.
In addition to what Barb said, check your Layers. Bring to Front will bring the Footer to the front of the layer it is on and may or may not solve the problem, as layers take precedence.
If it doesn't, then either:
1. resize the large object so it doesn't overlap the footer
2. reorder the layers so the footer layer is higher
Also:
Make sure the footers are on the footer layer.
Select All and examine the colors of the frame edges. Turn off Guides if you need to while doing this, as they can hide the frame edges.
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