Originally Posted by
HillRider
I also think the rear derailleur is getting in the way. Grab it and pull it back to clear the rear cogs and see if that frees the wheel to seat fully. Shifting to the smallest cog position as Crankycrank suggested will make this easier.
+1
HillRider has the right answer. And I would add that it will be easier to get the wheel mounted if you will turn the bike right side up and put the chain back on a chainring in the front. There is a tab on the derailleur next to the lower pulley. With the bike right side up, stand behind the bike and push the tab downward. This will help the derailleur clear the dropout plate and the axle should slide into the dropouts.