Sometimes fencers will give a guarantee of the quality of their work, other times not - depends who you use. I don't see that it would be a problem with the mesh, definitely the stobs. Where we are, getting the stobs into the ground is the hard bit, as the rock layer is very close to the surface, so it take ages with a pinch bar and trowel to get a deep enough support hole for the stob to withstand livestock pressure. You mention that they dug into the bank which might now be slumping, or the stobs might be less deeply in on the downhill side. If the fence is leaning away from the slope, towards the livestock, this could be what's happening. Whatever the reason, the fencers who did the work should have been able to assess the work and decide on the best way to do the job properly.
I would approach the fencers, not aggressively, and ask them to assess what's gone wrong, then ask what they are going to do to correct the problem.