So is she collected, "on the bit" and accepting of contact english consistently?
It's good that she consistently canters well, that's step one.
Loping is one of the hardest things physically you can ask a western horse to do. The amount of collection and correct carriage to lope correctly and also slow is immense. It takes a lot of physical conditioning to get to the point that it's physically possible. THEN we ask them to maintain all of that on a draped western rein.
Not being able to see what the actual problem is my suggestion would be to think "english" for awhile, but with a bit of a twist. Focus on consistent correct carriage working back to front, driving forward into your contact, with her rear well engaged and driving up under herself. Also work on speed control and rate. Make it so that you can get her to canter at any speed you want her too, canter fast, canter slow, canter at an average speed. Then put the two together so that not only can you rate her canter speed, but no matter what speed she's always willingly driving forward from behind and into contact with the bit. Then go western and work through it all again with western contact. At first you'll have to remind her a lot, and you probably won't be able to maintain the same nice slow lope that you could english, but eventually you work up to that same nice lope without the contact or consistent reminders.
Ask for rate changes western too. It's ok to "canter" western. It's better to always be able to control how fast or slow your horse is going than have a horse that thinks "I go THIS speed under a western saddle or I get in trouble". Plus it's easier to work on the rear end engagement when you are really driving them forward into a nice canter than trying to hold back the front end (slow down) while driving the rear end forward (which is essentially the contradiction in terms that a lope becomes).
