Control is a matter of will and belief in dreams. If you have doubt that you can do something, it will seriously effect the chances of it happening. You didn’t give much detail, but a lot of times we try to actively control the dream (i.e. you try to change the color of a DC’s shirt just by willing it). Passive control, while maybe not as exciting, is a better way to get results early on. Rather than me try to explain it, check out tosxyChor’s guide here.