A primer that successfully produces a PCR product but fails in Sanger sequencing could be attributed to several reasons:

Overall, the success of a primer in Sanger sequencing depends on various factors, including design, sequence compatibility, modifications, template quality, and experimental conditions, which may differ from those in PCR.

Sanger sequencing may fail because of the sequence of the PCR product, e.g. if there is mononucleotide stretch the sequence.

See point 3.

https://dnacore.mgh.harvard.edu/new-cgi-bin/site/pages/sequencing_pages/seq_troubleshooting.jsp

My first guess is that the annealing temperatures that you may have tried were sufficient for the forward primer, but not for the reverse primer. Also which polymerase are you using, and which annealing temperatures have you tried? Looking at the suggested annealing temperature for some taq polymerases from certain brands with your specific primers might require a lower annealing temperature than the recommended 45 °C...in these cases it may be necessary to switch to a different taq/phusion.

To test this hypothesis, it might be wise to run a reaction or two with just the reverse primer and see if you still get a bright band or not.

1. You may want to increase the length/Tm of your reverse primer.

2. You may also have a primer-template mismatch that works for PCR but creates problems during sequencing.

In PCR, amplification is exponential, involving two primers that replicate opposite strands. Each newly generated strand then serves as a template for further rounds of amplification. If a mismatched primer anneals to one of these extension products, it can still generate a perfect match during subsequent cycles, leading to robust amplification. This differs significantly in Sanger sequencing, which is a linear process.

In Sanger sequencing, the extended DNA strands are terminated randomly by the incorporation of ddNTPs (dideoxynucleotides), labeled with fluorescent tags. These terminated strands cannot serve as templates for further replication, which limits the sequencing process to just one round of linear extension per molecule. Consequently, primer mismatches that might be tolerated in PCR can cause significant problems in sequencing. Primers that perform adequately in PCR due to the cumulative effects of exponential amplification may yield weak or ambiguous results in sequencing, where each primer's efficiency directly impacts the quality and intensity of the resulting sequence data.

Extending your primer may compensate for the primer-template mismatch and increase your primer's TM into the mid-50C range. Another option is to use a degenerate primer. However, be aware that degenerate primers can cause as many problems as they solve. Be sure to double the primer concentration for each degeneracy.

I hope this helps.