How To Use Blast To Find Exact Matches Of Short Sequences?

Hi, I am using tblastn (under blast 2.2.25+) for exact peptide mapping (no gaps).
I want to map few peptides (about 6 to 50 AAs in length) to genome.
However, as I test a known peptide of 6 AAs,tblastn failed to mapped this peptide.
I have read the doc of blast, but failed to find a solution. What did I miss?
Thank you!
PS. I have also tried PGM (ProteogenomicMapping). This tool can map the known peptide tested above correctly, but it's slow in my computer which is impossible for large scale mapping.