If they were fresh and starved in shipping, it could take quite some effort to bring them back. Watch out so you are not using too little milk. In my opinion better more than less until they are brought back to life. This way you make sure they have enough food, you do not expect kefir first few batches anyway. If you put too little milk they might keep starving. If 5-6 batches are still not fermenting, then it is safe to assume they are done, but if you get any activity, few bubbles, yeasty smell, anything, then fight further as they will come back. If few batches and after 1-2 days for each there is no activity, milk remains milk, then they are done.
Also, if you’ve never had kefir, watch out that you don’t confuse kefir with spoiled milk. It will curd, whey might separate and it will smell kind of yeasty at the top.