Union-Boolean operation failed

Hello, This is my first time creating something useful. Looking for some help, I’m building an slideing rack insert that will hold an arm attachment which then will hold an ipad. The arm attachement will be slide through a rail rack. As for the attachment, this is what i created but am unable to union the 3 rows of cubes that slide into the rack and the above part that will hold an attachment piece, I tried to union before printing but I keep getting the error Boolean operation failed. Any advice will help!

If they aren’t intersecting or have mating surfaces, they won’t union. It’s hard to tell from the image alone, but I can’t think of any other reason.

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