Start with what diabetes does to a foot. Diabetes can damage the nerves and blood vessels in your feet. This damage can cause numbness and reduced feeling in your feet. As a result, your feet are more likely to get injured and may not heal well if they are injured.
That is also why it may not hurt. Diabetic ulcers are often painless, because of decreased feeling in the feet. The same goes for warning signs: redness, warmth, or tenderness are things to look for on your feet, but they are often absent because of nerve damage.
So no pain tells you about the nerves, not about the wound. A foot sore that doesn’t hurt is typical, not reassuring. That’s the reason to check your feet every day, rather than only when you feel something.
There’s also a cause that has nothing to do with how the wound is dressed. Foot ulcers are partly caused by too much pressure on one part of your foot. A wound kept under pressure is being re-injured, which is why taking the weight off it is part of treatment, not an optional extra.
Healing takes longer than most people expect. It may take weeks or even several months for foot ulcers to heal. Slow is not the same as not healing.
One sign bears on this question directly: wounds that are pale, purple, or black are less likely to heal. Contact your provider if your foot ulcer is very white, blue, or black.
Infection matters too. Poor blood flow can slow healing, so an infection may last a long time. Sometimes, a bad infection never heals, and the infection might lead to gangrene.
One more point applies to any wound not healing as expected. Very rarely, a wound that doesn’t heal as expected may be due to another condition, including skin cancer or inflammatory diseases. A healthcare professional may suggest taking a small skin sample — a biopsy — to be sure of the diagnosis. This is a routine procedure that helps make sure you get the right treatment.