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why there are no nucleotides in the position of the reference genome after the SAMtools Mpileup

I have been creating workflow which consists three main steps ( SAMtools View, SAMtools fadix and SAMtools Mpileup). Firstly, with the SAMtools View I filtered the input bam-file based on a bed-file that contains special regions of the third and fifteenth chromosomes. The input bam-file already aligned and sorted I took from the database. The bed-file was download from my computer and the firs line looks like this (3 193593144 193697811). After that the SAMtools Mpileup took file that contain only necessary chromosomes and the file with the indexed reference (as a reference, I used ucsc.hg19.fasta from the database). At the end of the workflow, I expect to see vcf-file with that contains information about the reference and alternative nucleotide of the third and fifteenth chromosomes. Unfortunately, I get the described file, but there are N in place of the reference allele.

Please help me understand what is wrong with my reference file