The D+ line needs to be pulled up by the device within 100 ms of connection to the host PC in order for the host PC to recognize the device.
If you are not using any external firmware load, then this is not a problem as the FX2LP default behavior is to automatically connect the pull-up resistor.
If you are loading the firmware from EEPROM on connect (boot options) then following things should be taken care:
1. If a "C0" Load is used (only the VID/PID/DID/configuration bytes):
Set the configuration byte in the EEPROM (Byte 7) to 0x00 / 0x01 i.e. The DISCON bit in the Configuration Byte should be set to 0, this will set the EZ-USB status to connected.
2. If a "C2" Load is used on the startup, then the device must load the firmware from the external EEPROM and set the DISCON bit in the USBCS register from the firmware to 0 before 100ms. In this case, to load the firmware fast, please set the first bit (bit-0) of the EEPROM configuration byte to 1 and make sure to set DISCON to 0 in the firmware after setting all the descriptors and configuration (this is done in the fw.c file of the firmware). Please use a standard example firmware like bulkloop.iic from the FX2LP DVK package in the EEPROM.
If you are using C0 or C2 load, you can try electrically disconnecting the EEPROM from the FX2LP I2C lines and check if the FX2LP will enumerate with default VID and PID as a NO EEPROM Device.
You can also refer to this Application note: FX2LP Hardware Design Guidelines