You may occasionally see 'TX Attempt failed (msg 18)' or 'CP busy' appear in the Messages field of proAIS2.
The B921/B922/B923/B924 all operate using CSTDMA - a ‘carrier sense’ system. They have to find a free slot to transmit in by listening just before transmission to check the slot is empty.
When a transmission is scheduled a block of 10 potential slots is selected. It will attempt to transmit in the first of these slots. If the slot is busy the ‘CP busy’ (meaning ‘candidate position busy’) message will be output. It will then try again in the next slot, and so on for the remainder of the 10 slot block. It is quite normal to see ‘CP busy’ messages in an area where other AIS vessels are operating. It simply means the first slot selected was busy. 99% of the time the transmission will be successful in one of the other 9 slots.
So long as the indicator remains green transmissions are occurring on schedule. If transmissions can’t be made because the AIS environment is simply too busy the indicator will turn amber.