Yes, it can happen that VPXA is not sending stats update to VPXD. It also can happen that VPXA did not get any stats from the host itself.
1. Login to the host (SSH) which is not showing any stats: and execute below command
/etc/init.d/vpxa restart (or stop and start)
2. how long you are getting these values as zero? Usually when host connects to the vCenter, it takes around 30-70 Sec to get stats displayed on VI client.
3. is your vCenter got restarted recently (right before stats are zero)? is DRS/HA also enabled on cluster?
Answer to above queries will help to take this forward