What is preventing my achilles tendon injury from healing?

See below: It could be many things but the most common is that you are not giving it enough rest.

Some studies have: Shown that if there was a gap remaining when a foot was positioned in the boot if treated non-operatively that there may be slower rate of healing or recurrent issue. This may be a criteria ofr opreative vs non operative treatment, if operative treatment then infection while not common but can be a factor to delay completion of treatment. All things being equal both methods of treatment can work.

Depends: It depends on the extent of the injury. The tendon has a delicate blood supply. Smoking can slow healing by reducing blood flow to the injured site. If completely torn it will take a long time to heal if treated conservatively. Surgical repair will allow tendon to heal faster with quicker rehab.