There are 2 main reasons why gaps come back after braces treatment 

1) Improper use of retainer 

Retainer needs to be worn regularly and is as important as your braces treatment, as your teeth have a tendency to go back to it's original place 

2) Eruption of wisdom teeth 

Many people undergo braces at the age of 12- 18 yrs of age.

Wisdom teeth usually start arriving at the age of 20+ yrs

In an attempt to create space for Eruption, it disturbs the alignment of the the rest of the teeth.

So what is the solution for it?

1) Use of retainer depending upon the severity.

2) Go for aligners/ braces treatment 

3) Getting rid of wisdom teeth 

4) You also consider cosmetics treatment for correction of minor gaps/ crowding