See User manual ection 7.2.17 Register file: 0x0F on TXFRS
The TX frame sent flag is set when DW1000 completes frame transmission. It can be cleared by writing 1 to the that bit in the status register, or by instructing another transmission.
If the TX flag does not get set, then you have some issues in the frame configuration and the DW1000 is not able to send a misconfigured/malformed frame and frame configuration. Please check that your SPI is working correctly.
Are you using our examples for example 1a/2a.
APS022 I sent you earlier would also describe this (section 5.4)