why kernel crashdump is not able to dump vmcore?

im using ubuntu 16.04 on esxi-6, the proc/cmdline shows crashkernel is loaded at boottime

BOOT_IMAGE=/boot/vmlinuz-4.15.0-54-generic root=UUID=b12d12f2-cfd9-465d-9b10b12d12f2 ro net.ifnames=0 biosdevname=0 text crashkernel=384M-:128M

there are no memory related errors on dmesg or syslog regarding crashkernel memory allocation ...

  /#kdump-config show
    DUMP_MODE: kdump
    USE_KDUMP: 1
    KDUMP_SYSCTL:  kernel.panic_on_oops=1
    KDUMP_COREDIR: /var/crash
    crashkernel addr: 0x29000000
    /var/lib/kdump/vmlinuz: symbolic link to /boot/vmlinuz-4.15.0-54-generic
    kdump initrd: 
    /var/lib/kdump/initrd.img: symbolic link to /var/lib/kdump/initrd.img-4.15.0-54-generic
    current state: ready to kdump
    kexec command:
    /sbin/kexec -p --command-line="BOOT_IMAGE=/boot/vmlinuz-4.15.0-54-generic root=UUID=b12d12f2-cfd9-465d-9b10-b12d12f2 ro net.ifnames=0 biosdevname=0 text nr_cpus=1 systemd.unit=kdump-tools.service irqpoll nousb ata_piix.prefer_ms_hyperv=0" --initrd=/var/lib/kdump/initrd.img /var/lib/kdump/vmlinuz

i have tried the kdump path to simple ext4 partitoin eg. /dev/sdc1 , nfs share and lvm partition but when the sysrq is triggered the server is frozen where i have to reset/force reboot the server to be back online. the crash storage directory has only one file kexec_cmd with content of kexec command in kdump-config show output.I see that there are some modules that needs to be blacklisted in MS-HyperV is there similar modules that needs to be removed on esxi ?

Thanks