Can you amplify any target genes from your samples? You could try something really conserved like actin or 16S rRNA just to be sure that your DNA extractions are successful.

How divergent is your strain from the reference genome? If the species has a large amount of nucleotide diversity between individuals you may need to try several primer pairs before you get ones that work.

You can first verify the primers if it is specific to your required sequence of SoxC. You can also run a gradient PCR for a set of temperatures to know the amplification temperature of your primers. It will help determine the optimal temperature for primer annealing and efficient amplification of the SoxC gene.

First of all, you need to confirm that you do have DNA and with a good quality, not fragmented. So you need first to amplify a housekeeping gene as Katei said, or like 16S and if it works you can use it as internal control later.

Also, I recommend you run your DNA on gel to see if it is contact or fragmented, since your amplicon size is big, this would be the reason why you couldn't amplify it.

Then, for the PCR reaction, you can try gradient PCR to find out the best annealing temperature.

Regarding reagent concentration, you should follow your master mix instruction, and for the primer concentration, it depends on the dNTPs conc. so they should recommend it also