Actually, it kinda was fixed. The only reason they were able to get there in the first place was because of the locked-on coordinates. With River there that time, there was no way to break back into the time where the Ponds were, and I'd bet it's even more of a fixed point once they get sent back the second time(To stay locked in the hotel).

Maybe Amy could've helped with locking on again, but for one, she wasn't exactly thinking straight, having seen her husband, just after they escaped death, vanish right before her eyes. That, and I don't know if the Vortex Manipulator would've gone with her. Theoretically it would've, but..