These are probably obvious but
Depending on your scale method there is one axis that will always "fit" and the other will be too short.
You need to ensure that on your layout that you extend that axis.
Your HUD should be scale 0% and parallel 0,0
I have never used scale inner I tend to use scale outer or letter box scale.
You need to test it on various devices to confirm. I use a Sansung S3/S5 vs Nexus 7 and a Samsung Tab.