Test suite collection on Selenium Grid

I have a test suite collection having 3 test suites with 4, 9 and 6 test cases.
Using Selenium Grid I started the execution and see that even after passing max concurrent instance on 5 node (with 5 browser threads enabled) only 3 nodes are up with 1 browser on each node.
So I set 15 concurrent instances but the number test suites are 3 that means there are 3 test suites can run at the same times. Not 15 test cases
Ideally test suite collection should combine the number of scripts and distribute it to each node as browsers are available.
For me total 5 nodes with 5 threads so total 25 browsers are available.
Please suggest what can be done.