First make sure the Arm and Enable bits are set in the Endpoint Control Register [03H, 11H, etc.. ]. If the Enable bit isn't set then all transactions to that endpoint will be ignored.
If the SL11H (slave mode) or SL11 still isn't responding to packets/requests then read the Packet Status Register. Pay attention to the time-out bit (bit 2) and the Error bit (bit 1).
If a time-out ocurs then make sure the direction bit (bit 2) in the Endpoint Control Register [03H, 11H, etc] is set appropriately. If this bit is set to expect an IN, and an OUT occurs, then the part will ingore it and the time-out bit will be set (and vise versa). If this is the case, re-set the direction bit appropriately and re-arm the endpoint. The host will resent the packet upto three times if it is ignored.