The journal run under PKP OJS platform for 3 years, never has problems in indexing in Google Scholar, I recently found they removed part my article in Google Scholar, mainly all articles published in 2018, where the articles in 2017 and before still searchable.
In fact, technically, all papers published and run under OJS will be automatically be indexed by Google Scholar, I check the plugin in OJS admin setting, the tick is still valid in “Google Scholar Index” Plugin.
I wonder why and I had sent email form to Google-scholar support, but so far, no reply.
Anyone running journal site and having this problem appears so far, hope to get some advice and ways to solve.