Carpet on stairs loose
I have found that in the past getting the carpet wet with warm water helps it stretch. Just like when you shampoo the carpet, it gets loose and when it dries it shrinks back somewhat.
Make sure that you put a fan on it to help the drying process when you are done. There are also carpet stretching tools available at rental stores. Some good carpet stretching hand tools are out there for small jobs as well.
Pay no attention to the negative comments entered by that Joba the bonehead Fett. I have been perusing the chat rooms and have found that so far he has offered nothing of value or help. He can only critisize and post duragatory responses like he did to you and does to everyone else. I don't know who elected Joba the chat room sheriff but I know I did not vote for that loser. He may post like an idiot and reply like an idiot, but don't let that fool you. He really is an idiot!! He seems concerned only with his opinion and not the good advice I've seen given by others on this site.