Why are my Nextflow processes not executing in parallel?

I have written a Nextflow script with three process:

• The first process takes a pair of fastq files and aligns with reference genome. The process writes the resulting SAM file into sam channel.
• Second process takes input from the sam channel and creates a BAM file from it, and writes it into bam channel.
• Final process reads from bam channel and sorts the BAM files.

Following is the entire code:

#!~/bin nextflow

reads_ch = Channel.fromFilePairs('raw/*_{1,2}.fastq.gz', flat: true)
ref_genome = file('reference/genome/index/ref_genome')

process mapToRef {
    memory '20 GB'
    cpus 16
    input:
    tuple val(sample_id), file(read1), file(read2) from reads_ch
    
    output:
    tuple val(sample_id), file("${sample_id}.sam") into sam_ch

    script:
    """
    ~/biotools/bwa/bwa mem -t 60 ${ref_genome} ${read1} ${read2} > ${sample_id}.sam
    """   
}

process samToBam {
    memory '20 GB'
    cpus 16
    input:
    tuple val(sample_id), file("${sample_id}.sam") from sam_ch

    output:
    tuple val(sample_id), file("${sample_id}.bam") into bam_ch

    script:
    """
    ~/biotools/SAMTOOLS/samtools-1.12/samtools view -S -b ${sample_id}.sam > ${sample_id}.bam 
    """
}

process sortBam {
    memory '20 GB'
    cpus 16
    input:
    tuple val(sample_id), file("${sample_id}.bam") from bam_ch

    output:
    tuple val(sample_id), file("${sample_id}.sorted.bam") into sorted_bam_ch

    script:
    """
    ~/biotools/SAMTOOLS/samtools-1.12/samtools sort ${sample_id}.bam -o ${sample_id}.sorted.bam
    """
}

All three processes successfully execute their commands. However, they are executed sequentially. For example, the first process mapToRef runs sequentially for all read pairs, followed by samToBam, which too, runs sequentially after all mapToRefs have finished execution. I was under impression that parallelization would “work” out of the box, but I must be doing something wrong. I am a complete beginner in Nextflow and would greatly appreciate any help regarding this.

EDIT

Output of reads_ch.view():

[ERR2512393, /home/work/raw/ERR2512393_1.fastq.gz, /home/work/raw/ERR2512393_2.fastq.gz]
[ERR2512394, /home/work/raw/ERR2512394_1.fastq.gz, /home/work/raw/ERR2512394_2.fastq.gz]
[ERR2512391, /home/work/raw/ERR2512391_1.fastq.gz, /home/work/raw/ERR2512391_2.fastq.gz]
[ERR2512392, /home/work/raw/ERR2512392_1.fastq.gz, /home/work/raw/ERR2512392_2.fastq.gz]
[ERR2512390, /home/work/raw/ERR2512390_1.fastq.gz, /home/work/raw/ERR2512390_2.fastq.gz]