What to do if I have a foot ulcer that is close to healing, but the last little bit will not close.?

Foot ulcer. Work with your team at the wound center or clinicians. You have not given enough information to assist us in helping you, as different ulcerations are due to different causes. For example, vein ulcers are best treated with compression; diabetic ulcers ultimately need offloading with casts or boots.
Change current Rx. Change the current treatment - on the final part of healing avoid alginates or foam dressing these prevent skin closure. Orally take Niacin 500mg daily and vitamin k2 100mg daily. Niacin stabilizes vasculature in that area and accelerates healing and vit k2 helps with mmp a major factor in tissue destruction - with the above I have seen would heal within a week.
Barrier to healing. It is not uncommon to reach the stage where you are. When the cause for the wound and the barriers havent dealt with we would have troubles down the road and even when it closes it may come back.