The variable in your message does not match a column header in your file.

Check three things:

1. The header is written exactly as $CUSTOM1$, with both dollar signs and no extra spaces.
2. The same spelling is used in the message text.
3. The column actually has values in it. An empty cell leaves nothing to substitute.

Use Generate First Line Preview before sending. It shows the first row with values filled in, so an unreplaced variable is obvious straight away.