There could be a couple of reasons - with any deep-frying, it's important to make sure your oil is staying at the right temperature. If it's too cold, the batter will turn out soggy, but if it's too hot, it can burn the outside. This is when I HIGHLY recommend a thermometer so you can easily monitor it. I use a cheap probe thermometer from Amazon, which I also use for cooking meat and it's one of my most used kitchen tools. Crowding your pot can also result in soggy batter, and this again is because adding anything to oil will lower the temperature, so adding too much can drop it too far. So fry in batches and keep checking on your temperature for the best results.