Buckling carpet

Hi There, I paid $2800 for carpet and underlay in August last year. My tenants were loathe to complain so didn't tell me when it started buckling all over the house. The carpet company had just changed hands and it was obvious they were in a bit of turmoil/teething. I'd searched high and low for better quality carpet at a 'reasonable' price. I have recently discovered the issue and contacted the carpet company. They suggested humidity and then gave me the number of a guy who could rectify at a cheap price. I wasn't very happy with the this response. The previous carpet lasted 18yrs with no humidity issue. I believe it was poor laying that is responsible. Is it reasonable to insist they send a technician to restretch the carpet to walls? I have pics to show issue more clearly. My tenants are in their 70s and have rugs down so it definitely wasn't misuse. Just want to know if I am justified in insisting the carpet company sort it out.

Pics here: http://imgur.com/a/sX1Zg

The carpet seller is at fault here for hiring a tradesman who does use power stretcher during the install. You will need it power stretched as the layers only used a knee kicker. Finding a layer who has a power stretcher is the tricky part and getting someone to pay for it is even harder.