regular nmap scan flaky; hosts are missing sometimes

I'm using nmap 7.6 to scan my local wired network every minute.

The problem is that the scan results are flaky: Sometimes hosts are missing in the result, although they are definitively available (e.g. the router that I use to get into this network). A minute later, the same scan finds the machine again.

My nmap command is:

nmap -sS -n -p T:21,22,80,443,2101,9009\
     -oX "/tmp/nmap-`date -u --rfc-3339=seconds`"\
     192.168.178.0/24

nmap reports hosts as "up" in the XML file because of ARP responses:

<status state="up" reason="arp-response" reason_ttl="0"/>

The hosts that are reported as "missing" change from case to case; it's not always the same host that is reported missing.

My questions are now:

1. Why does nmap not detect the hosts as up?
2. What can I do to improve reliablility of the scans?

Related:

• https://security.stackexchange.com/q/191318/25645