A couple of things that haven't been mentioned, firstly if a salmon is in the mood it's far easier to catch than a trout, if it's not you are on a loser before you start. The fresher a salmon is the more likely it is to take a bait, or immediately after a rise in water levels can bring them on when the level has started to fall again.
If a salmon is following but not taking it's clearly interested so you need to make some small changes, first change the size of the bait, usually smaller is the way to go, then try a change of colour. When we fished the Wye in the good old days we knew we had fresh salmon in the pool as we could see them and we might be using our standard 2½" wooden brown & gold minnow. We might show this to a fish a dozen or more times then switch to a 2" and immediately get a take. The great unknown is of course whether that fish would have taken the original minnow on that cast, we'll never know but a take immediately after changing the bait happened too often to be just chance.
If a fish is following your lure do not slow up the retrieve to let it catch the lure, if anything speed up to make it look as if it's going to escape.
It took me about a dozen tries to get my first salmon on what then the most prolific river in England or Wales so don't give up, it will happen.