there are a couple of possibilities here. Does the lever feel spongy ? If so there is probably air in the system and you haven’t bled them properly – time to have another go.
does the lever feel good with a relatively solid feel when you pull the brake? If so the problem might be a contaminated disc. You need to clean the disc, you might just get away with a totally clean rag and then getting your brake nice and hot, but more likely you will need some disc brake cleaner or some alcohol.
worst case is that the pads are contaminated now as well, sometimes this improves with use but they might need changing.