Hi 3image - currently the merge command only works on edges, not faces. If you want to merge coplanar faces though you can separate them into individual surfaces, then select them and run boolean union.

For edges there are some restrictions on what can be merged, only a juncture between 2 edges where the edges meet smoothly can be merged. Probably in your particular case it's not meeting those restrictions. If you think that you have a case where some edges should be mergable but it isn't working please post the 3DM model file and I'll take a look.

- Michael