The h-index does not increase linearly with citations and has nothing to do with other research scores. It is the number n of publications that have at least n citations. In ResearchGate, your h-index is 5. To get an h-index of 6, you will have to have 6 publications with at least 6 citations. However, usually citations are not distributed equally over all publications, but concentrate on a few. One of your paper (https://www.researchgate.net/publication/357993740) has 30 citations, but others have not yet been cited (or ResearchGate did not find these citations).