A question regarding AI pathing

[Skaarphy]

I have built a semi-underground base with 36 blocks length and 15 blocks width, and with one block being above ground and five blocks being below ground. There is an entrance at the end of one of the long sides with double stairs leading down. The below ground part has a two block width pathway with electric fences lined up all the way from one side to the other. 

The idea is to lure them down the stairs onto the pathway where they get electrocuted and where I then blast them with explosives. Only it doesn't work. 

What they do instead is run all across the above ground section to the side where I'm at and start destroying blocks. It's rare that one of them actually takes the stairs. Anyone an idea why that is?

 

[Beelzybub]

Maybe the path is too long. Or obstructed in some way that they don't see it as a viable path.

 

[Skaarphy]

I was wondering whether the path is too long: They spawn near the side where I'm at, don't register the entrance on the other side, and try to dig through. But they do this no matter where they spawn. I have since made a second entrance, on the diagonally opposite side of the first one. When I'm on one of the sides and lock the door behind me they'll go and attack the blocks on my side. But when I then open the door, run over to the other side, and close the door there the ones that don't immediately come inside will then run over and again attack the blocks on the side where I am. I can then repeat that, always with the same result.  

I've made a few screenshots of the base. The first shows how the entrances look like, the second the below ground part, and the third the above ground part.