The key to getting that perfect crunch is taking your time when you cook the chicken. Time to make sure all the fillets are coated properly, time to heat the oil to the right temperature and time to cook the burgers in batches if they don't all fit in the pan. The main 3 reasons why your chicken might be soggy are:
• Your oil was not hot enough. This is undoubtedly the most common culprit. If you don't have a kitchen thermometer then I strongly recommend investing in one, they are very inexpensive. The chicken needs to sizzle on contact with the oil and after 10 minutes it should have turned a golden brown colour.
• You didn't fully coat the chicken. The burger needs to be fully covered in flour. You won't need the entire amount of flour indicated in the recipe, but adding the chicken to a large bowl of it will make it easier to evenly coat it.
• You overcrowded the pan - when you add the chicken to the pan it reduces the temperature of the oil. The more chicken you add the more the temperature will reduce. Furthermore, if the oil can't move around the chicken and the pieces overlap then those parts won't become crispy.