Western pleasure lope question?

okay so my horse is a great western pleasure horse,but i have one problem with him. i can not keep him on the rail at a lope. he works just on leg ques and when i ask for the lope for some reason he will not stay on the rail. i have tried bumping squeezing and fanning my leg on his side but he does not respond, and if i give any tap with the spur he changes leads. so if you have any tips to help keep him on the rail it would be greatly appreciated. oh and also i ride him one handed and my last resort is pull him to the rail with my other hand.