[EDIT: as Jai points out, the problem was likely forgetting IPv6. The AAAA record is now updated, and I should be able to tell in a few hours whether that fixed it.]

I'm helping my dad migrate some code to a new server. It was at 162.209.99.139 for years, but ten days ago he changed his DNS settings to point to 34.199.143.13. Everything looks good to me:

$ dig notes.billingadvantage.com
notes.billingadvantage.com. 1800
    IN    A    34.199.143.13

The TTL is 1800s, or 30min, which agrees with dns.google. I expected everyone would be moved over within a couple hours, but a week later the old server is still receiving traffic nearly as much as the new:

date	old server	new server
2021-02-05	943	0
2021-02-06	201	127
2021-02-07	17	108
2021-02-08	364	423
2021-02-09	488	448
2021-02-10	255	503
2021-02-11	281	345
2021-02-12	250	248
2021-02-13	0	88
2021-02-14	0	78
2021-02-15	217	262
2021-02-16	202	287

The old server getting no traffic on the 13th and 14th is probably because that's the weekend, and the users who happen to be still stuck on the old site aren't using it on the weekend. I asked one of the users still getting the old server to try rebooting, to no effect.

I thought maybe something was misconfigured with the name servers, but it looks fine:

$ whois billingadvantage.com \
   | grep 'Name Server'
Name Server: NS1.ZEROLAG.COM
Name Server: NS2.ZEROLAG.COM

$ dig notes.billingadvantage.com \
   @ns1.zerolag.com
notes.billingadvantage.com. 1800
   IN    A    34.199.143.13

$ dig notes.billingadvantage.com \
   @ns2.zerolag.com
notes.billingadvantage.com. 1800
   IN    A    34.199.143.13

I'm not seeing any references to the old IP address anywhere.

Any guesses about why the traffic isn't moving over?