Using the default keybinding code under Advanced Options. Have a couple flying mounts set at priority 1 (none are any higher than 1), including Amalgam of Rage. When pressing the keybind many times over in a flying area, all the flying mounts with priority 1 are seen except Amalgam of Rage.

"/cast Amalgam of Rage" in chat box works.

If I disable all other mounts, it does not summon. I get the message "You can't mount here" on the screen.

Hmm, weird. Unfortunately I don't have this mount and probably never will.

Can you go somewhere that it works and paste or screenshot me the output from: /litemount mounts Amalgam

Also if you can make it fail then send the debugging info: https://github.com/xod-wow/LiteMount/wiki/Report-Bug

"/litemount mounts Amalgam" output:
LiteMount: --- Mount Dump ---
LiteMount: Amalgam of Rage
LiteMount: spell: Amalgam of Rage (id 369476)
LiteMount: flags: DRAGONRIDING,FLY (default DRAGONRIDING,FLY)
LiteMount: mountID: 1596
LiteMount: family: Gravewing
LiteMount: isCollected: true
LiteMount: isMountable: true
LiteMount: isFavorite: false
LiteMount: isFiltered: false
LiteMount: priority: 1
LiteMount: castable: true (spell true)
LiteMount: isUsable: true
LiteMount: mountTypeID: 424
LiteMount: sourceType: 8

Solved: I had a rule set up from a previous profile that I copied my current one from that was limiting mounts to a group that did not include Amalgam.
Major goof on my part, I apologize. Thank you for the help!

Aha! No problem, glad you found the answer.