I broke my foot a month ago, and during my last doctor's visit, he said that there was no sign of healing. what could be causing this?

Non-union: A fracture may fail to heal/unite for several factors together or alone. We call this a non-union. Some factors include smoking and walking on the foot. Certain bones in the foot have a higher chance of non-union. If the original fracture was was greatly displaced the the two bone may not be able to unite.

Fracture: As stated, there are more than a couple reasons why it may not be healing. Ask questions of your doctor and check to ensure you are doing everything that is being asked of you as a patient. This greatly helps in having successful healing.