Details:when you had hotplug vm(guest os :linux like redhat 5.9 and below or windows 2003) mem or cpu property done in vCenter.The guset os can't recongnize the hotplug part.
Solution:
hotplug mem
grep line /sys/devices/system/memory/*/state
echo online > /sys/devices/system/memory/memory[number]/state
hot plug cpu
echo 1 > /sys/devices/system/cpu/cpu*/online
grep "processor" /proc/cpuinfo
PS:windows 2003 and below should reboot guest os to recongnize the hotplug part.
linux 6.x and UP ,windows 2008 and up can automatic recongnize .