Apparatuses, methods, and systems are disclosed for transmitting UL data in an access network using unlicensed spectrum. One apparatus includes a processor that performs a first LBT procedure for transmission of a first TB associated with a first UL grant using a first HARQ process, where the first LBT procedure results in LBT failure, the first TB being stored in a transmission buffer of the first HARQ process. The processor also performs a second LBT procedure for transmission of a second TB associated with a second UL grant using a second HARQ process. The apparatus includes a transceiver that transmits the first TB using the second UL grant associated with the second HARQ process in response to the second LBT procedure resulting in LBT success.