If you have stained them only once, stain them again. Allow 10 – 15 minutes to pass after staining before wiping. This should help considerably.
If staining a second time doesn't go as dark as you want then you will need to use a gel stain. Gel stain is a very different type of stain when compared to a wiping stain. It is possible to get a very dark or black color but some on the wood grain will be lost. Try applying a thin coat of gel stain then wait 24 hours before applying another coat.