Gremlins, or maybe the cats (if you have any).

Make sure you have plenty of capicitance on the VUSB pin. (.22uF to .47uF)
And have both a 10uF and 0.1uF capacitors on the Power Rails (VSS-VDD).

Plugged, RX_READY and TX_READY are just normal "Bits".

RX_READY indicates that data has been received. It should be cleared after use.
Code:
Main:
    IF RX_READY THEN        ; USB data has been rcvd
        RX_READY = 0        ; clear the Ready flag
        USBIN 1, Buffer, Cnt, Main
        LCDOUT $FE,1,STR Buffer
    ENDIF
TX_READY indicates that the device is plugged in and the SIE is ready to accept data to send.
Code:
    IF TX_READY THEN         ; USB is Ready to transmit
        TX_READY = 0         ; clear the Ready flag
        ADCIN 0, ADval
        ARRAYWRITE Buffer,[DEC4 ADval," "]
        USBOUT 1, Buffer, Cnt, TXfail
      TXfail:
    ENDIF
GOTO Main