Generally, avoid any contamination with any bacterial DNA. it depend on type of reagents that you use, some reagents buffers optimized to make amplification to GC rich genomes, if not specified you just add DMSO at percentage of 5% to your PCR and it will work great.
You need to work initially with the prescribed conditions for such organisms, then you can modify the conditions depending on the fingerprints that you will get i.e. reduce/increase template, reduce/increase Mg2+etc. You might need to post an electrogram you have obtained and then we can note what can be of help. Or alternatively you might need to check on PCR parameters, annealing, denauration etc conditions. They do affect the amplification quality and quantity.