Here is a dungeon map I made recently. I have used an Outer Glow instead of an ordinary glow in this case, and then a Blend Mode instead of a straightforward transparency. The Outer Glow is very similar to the Glow, but has slightly different properties that allow it to be used in conjunction with the Blend Mode. The Blend mode works in a similar way to the blend mode setting on a layer in PS or any other bitmap app.

You could copy these effects across from my map to your own and play with them if they aren't quite right.

I've also posted a rendered export of this map, in case you had any red Xs. The style is Dungeon's of Schley - SS4

People often apply a glow effect to the GRID sheet and edit the effect until it looks right.

Nice map, by the way! :)

1" = 5 feet = 1 grid square.

Set your worries aside for a few days. Nothing is worth ruining Christmas over.

Just checking that you aren't importing the image as an image, and not a symbol.

Check that you haven't called the one that's missing exactly the same name as one of the existing symbols.

Remy just suggested - double check that it really is a png image, and that the length of the filename doesn't exceed 32 characters.