Hello,
The first suspect would be the automatic mode. If you are shooting in the automatic mode you may want to switch the camera to a more manual or the cinema shooting mode to allow for more options. Please also be sure none of the settings listed on page 51 of the user manual are set as they can get in the way of the slow and fast recording modes.
http://gdlp01.c-wss.com/gds/9/0300011399/03/xa20-25-im4-n-en.pdf
Lastly, although it is likely based on today's available cards that your memory card already meets this need, you will want to verify the memory card you are using has a speed rating of at least class 10.
