merge samples

Hi
I have only 4 samples, but each sample has 23 fastq file. So I gernerate 23 gVCF for each sample by GATK-3.7.
Finally, I got VCF file by GenotypeGVCFs. However I found that most of snps in VCF file their genotyes and DP are the same as the 23rd gcvf for each sample.
Do you know which step may influence the result of merge these 23 gvcf per sample?

For example:
my commend line for AddOrReplaceReadGroups
RGID=sample1_023 RGLB=v37 RGPL=illumina RGPU=barcode RGSM=sample1

Thanks for your help.
Wendy