The 6th trilateral summit was held on 1 November 2015 in Seoul, resuming the summit since 2012 due to a variety of disputes and issues ranging from World War II apologies to territorial disputes among the three nations. During the summit, Chinese Premier Li Keqiang, Japanese Prime Minister Shinzō Abe, and South Korean President Park Geun-hye agreed to meet annually in order to work towards deepening trade relations with the proposed trilateral free trade agreement. They also agreed to pursue the six-party talks over North Korea's nuclear weapons program.
It mentions there were tensions because of Japanese provocations against South Korea and China over various historical and territorial issues, but then a meeting was held in 2019, and ever since then there has been no meeting.
Why hasn't a trilateral summit held since 2019?
The initiative of China-Japan-South Korea Trilateral Meeting was started in 2008 in Fukoka, Japan. The proposal was put forward by former South Korean President, Lee Myung-bak to have a platform aside from the framework of ASEAN Plus Three forum to initiate a better understanding of one another in a strategically conflict ridden and economically volatile region. This led to the establishment of the Trilateral Cooperation Secretariat in Seoul in September 2011 later. From the year of its establishment till 2012, five Summits have been held. But due to disputes ranging from the apology by Japan to China for atrocities committed during World War II, the comfort women issue between ROK and Japan, to rising tensions that led to significant deterioration of relations between the Northeast Asian nations, no Summit took place after Japanese Prime Minister Abe took office in late 2012.
The same thing again, but it doesn't explain the gap between 2019 and now specifically.