Without seeing it it could be due to any number of issues. And none of them are related to the constant chord of the wing.

For starters do you have the anhedral and forewing mounted landing gear as per the original or is that forewing flat?

How much dihedral does it have in the rear wing?

What are you using for control surfaces? I'm thinking that if your "elevators" are flaps on the forewing then increasing the camber and angle of attach by deflecting them down may be part of the problem. Not that you can do much about that.

Got pictures? What am I saying... Of COURSE you got some pics. Flash 'em up and let's see what this deamon of the netherworlds looks like.