I have a brother sewing machine. i threaded and have rethreaded and threaded the bobbin over and over. it will not move the fabric and botton thread will not catch onto top. what am i doing wrong?

Brother machines are usually a very good brand. Sounds like you are having a small problem with threading..........I know how frustrating that can be!
Start over....totally from the beginning. First, clean the machine of all dust, I use a small paint brush to remove lint from the bobbin case area. Oil the machine. Remove all the thread from the machine, use a different bobbin and rewind it with new thread. Make sure the bobbin thread is the same as the thread you use to thread the upper needle with. Use a new needle that is the appropriate size for your sewing project. Check to make sure the feed dog is moving when you turn the wheel that raises the needle up and down. New needle, new bobbin, new thread......before you place the fabric under the presser foot (and with the presser foot raised), turn the wheel one complete rotation to allow the upper needle thread to catch the bobbin thread. Gently pull both threads out towards the back of the machine, making sure the bobbin thread is now coming up through the little hole in the feed dog plate. Place fabric under the presser foot, lower presser foot and as you slowly start to sew, make sure you have the bobbin thread pulled with slight tension out from the back of the needle. This will take up any slack in the bobbin thread and keep it from making a knotted mess. Good luck, I hope this helps.