This may or may not be a developmental issue.
Consider that most children are ready for potty training sometime between 18 months and 3 years, but no all children are. Some children, with regard to potty training may not be psychologically or intellectually ready.
Even with children who are ready, there may be resistance to the training. This is because, psychologically, the free reign of the 2 year old is a pleasing experience, and potty training is moving them to unfamiliar territory. Also the child could have had a bad experience that is delaying training or causing relapses.
If your child is resisting training, I recommend you stop for a week or two and then try again. If you try to persevere, and force it, you may not be happy with the results or the effects on the child.
You can read more about potty training in situations like yours at this link: