Over time, you should vary all aspects of the hides besides the odor itself.
For example, don’t just use metal tins. You should practice with hides made of many diverse materials: steel and burlap, wood, cotton, water, plastic, glass, etc..
You should also vary the ageing of the target odor. Sometimes you may place the hide and start searching within 5 minutes. Other times, place the hide and go back a few hours later to perform the search. That way, you are prepared for competitions where you are the first competitor, and the last competitor after a long wait, and your dog will perform well regardless.
Another important variable to play with is the environment of the search area, including wind changes, weather conditions, and distractions. Your dog should be able to search in the rain, and if there are cars driving by. This generalization to all environments is a big job that will take a long time. In each training session, I like to do an easy search, followed by a challenge (with a corresponding reward), then finish with another fun easy search.
At some point, you have to test your dog to ensure that he is really detecting the target odor. It is fairly common for people who have only trained by themselves to come to a group class only to fail the first time time the dog searches for hides made by a new person. Prior to that test, the dog may have been relying on smelling the handler’s scent, the scent of their home, the scent of the container, and the scent of their food on the hides, in addition to (or instead of) the target odor. When the familiar scents are removed, the dog may be confused about which odor that they should find. So when you’re training by yourself, be sure to touch all the boxes, and clarify for your dog that he’s not rewarded for finding your scent. Ask to borrow a friend’s hides, or better yet, have a training partner put out hides for you.
In the long run, when you systematically vary everything except the target odor, you and your dog will be far more confident and prepared about your nosework skills.