TX Frequency must EXACTLY match the RX frequency on the remote side. Type freq command to view. Verify that the frequencies used match the regulatory body (FCC) approved frequencies.
Opmode is off. Run opmode command to view, and run opmode on to enable. If the unit was rebooted and the default opmode is not on, the unit will come back on but the opmode will be off. To prevent, set default_opmodeon and save or config save.
Are the frequencies configured correctly on each radio (TX and RX)? High frequency on the -B model and low frequency on the -A model (for licensed links). The TX frequency must match the Rx frequency on the other side.
Apex & SP ODUs only: Is the waveguide transition installed properly? The rectangular openings on the ODU and transition must be lined up, and the transition tightened properly. Remove and reconnect if not. This is shown in the manual.
Are both ODUs mounted on the same polarization? Licensed radios must be mounted with matching polarizations at each end of the link. If one side is mounted on the antenna differently than the other side, the link will not lot or the signal level will be 30 or more dB lower than expected. For SL24, one unit must be installed with the V up and the other with the H up, as this is a cross polarity design. From the ground you can tell by ensuring that one unit has the handle up and the other has the handle to the side.
IF ACM or rateshift (Giga) is being used, it may not be enabled on both ends of the link. Enable the ACM and reset the speed on each end. The speed must be set the same on both ends.
If RSSI is present, but the system is not linking on one or either side, try setting both to a lower speed (bandwidth/modulation) that will be more robust, for example 10 MHz QPSK to see if it will link there. If it links at a lower modulation, but not at a higher speed, please see high MSE troubleshooting.
Check status pll to ensure that all the PLLs are locked.