Pi-Hole works for a short period and then tapers off to nothing

This does not guarantee that DNS traffic will go to Pi-hole. Pi-hole should be the only DNS server provided to clients.

Please post screen captures of the following things from your Pi-hole web admin GUI. In this forum, you can directly paste images into your reply.

The current query graph from the dashboard. Should look something like this:

The long term database query graph (under long term data) for the past week. Should look something like this:

And then from the Pi terminal, the output of the following commands:

This will be the total number of queries in your long term database since you installed Pi-hole:

sqlite3 /etc/pihole/pihole-FTL.db "SELECT value FROM counters WHERE id = 0"

This will be the number of unique domains queried in your long term database

sqlite3 /etc/pihole/pihole-FTL.db "select domain from queries group by domain order by domain" | wc -l