From what I hear, the Kaveri engine suffers from similar issues that Chinese engines do, they degrade far too quickly. More specifically, from what I've read, parts of the engine, in particular the blades, tend to crack under pressure. Normally, that wouldn't be too much of a problem, as even the F-35's current engine have had issues such as this, but the Kaveri (much like Chinese engines) have this issue at an unsatisfactory level.
Again, I'm just paraphrasing an Indian member here (forget his name, though I don't think he comes here anymore). From what he told me, the reason why the Kaveri has a thrust issue is precisely because if the engineers push the engine to do to much, it literally cracks under pressure, which is why it is under powered for the Tejas.
AGAIN, I'm not an expert here, I'm paraphrasing. If anyone can correct this information, please feel free to do so.