Hi Alex, welcome to the forums! It's hard to troubleshoot without the actual recipe, but from what I know there are many reasons that can cause a dense or soggy banana bread:
- Too many liquids: are you measuring your ingredients by weight or volume? Volume measurements of flour aren't precise, and if the recipe just said "X bananas", you could have easily ended up with way more than intended as bananas naturally also vary considerably.
- Related to the point above, sour cream or yogurt can especially make for denser banana breads.
- Incorrect amount of leavening: too little means not enough rise, but too much can also cause it to collapse.
- Overmixing (which is especially easily done in the muffin mixing method) creates too much gluten, toughening the product and inhibiting rise.
Another thing to remember is that banana bread is a quick bread, so it's not really supposed to be light and fluffy, although I'll admit many banana bread recipes out there are closer to cake than bread. If that's what you're going for, then you could just search for a banana cake recipe - nothing wrong with that!