must be few things...
  1. Wich PIC are you using?
  2. wich PIC pin are you using?
  3. are they multiplexed with analog stufff???
  4. if so did you disable them prior to use them?
  5. Is your datain is before of after the main loop?
  6. If before did you place a Jumpstart Goto to jump over your SERIN?