While battling another Hover Tank it can hit my immune tank. But I can't hit the other Hover tank unless I am next to it. Am I missing something. A Hover tank can hit me from 5 or 6 spaces away while immune but I can hit a immune tank.
It's always possible there is a bug but given the sheer amount of times I've used hover tanks this last week it seems unlikely it's a general bug. More likely you are mistaken that you are on the plain or the unit in question can see through stealth. As noted above Rail guns ignore stealth so they can always attack your unit. It's also possible if you aren't being careful to move your hover tank more than you think when if you are clicking on the enemy to attack it. I've done that a couple of times where I stopped on the plains clicked on an enemy to target it and them my hover tank moved in order to be able to shoot it (and the next area was not plains).
If this is truly what has happened, you should contact in-game support. I don't know if they can replay battles or if you would need to try to get a screenshot of it happening, but that is the avenue you should pursue. I use Hover Tanks frequently on two worlds and have never had one hit from range by a unit unless it can ignore stealth. Which other Hovers cannot.