if it's the front tank bolt that won't go in ( and no obstructions as you suggest ) then take the 2 rear pins out that the tank hinges on, and try then , tank is a pig to line up..... the hinge pins will be a pig to get back in too, but not as difficult as your front at the monent!!!
Thats why slotted ninges are available , to move the tank back slightly so it lines up with the front mount/ bolt...... moto hooligan does the slotted hinges... you get the idea if you see them.....
I took all the bolts out on mine if removing the tank..... made it easier. but in hind sight the slotted hinges would've been great...
I'm sure some peeps on here have taken a file to the mounting holes of the hinges to get a bit more movement back over....
good luck