Samtools and bcftools: Why aren't some of the calls "missing data".

[molecules]

I'm using samtools mpileup and bcftools view to find variants. I'm perplexed why none of the alleles for any of the samples are shown as "missing data" (e.g. "./."). The coverage shouldn't be deep enough nor the samples similar enough for there not to be at least some "missing data". Here's my sample code:

samtools mpileup -uf ref.nfa A.bam B.bam C.bam  | bcftools view -vcg - > ABC.vcf

Is there a flag I'm missing or misusing?

This is using samtools version 0.1.19-44428cd and bcftools version 0.1.17-dev (r973:277) on CentOS 6.5 (64-bit).