What's wrong with Irrefutability and Defeasibility Proposals to Gettier Problems?

Gettier problems are cases, or examples, which appear to run counter JTB analyses of knowledge as justified true belief. According to JTB theory of knowledge, S knows p, only if: a) The proposition p in true; b) S believes that p; c) S is justified in his belief that p. Gettier problems state that the three conditions (a-c) are not sufficient for explaining what knowledge is. Because there might be some counter examples, which are cases of justified true belief but are not considered as cases of knowledge. Most of the epistemologists have accepted Gettier’s conclusion and tried to find an alternative theory of knowledge. Many of these epistemologists have tried to add a fourth condition to the earlier three conditions (a-c), to get a Gettier-proof definition of knowledge. Accordingto one of these strategies, the added condition is: d) the epistemic justification for truth of his belief is not a refutable one. And according to another strategy, the added condition is: d´) the epistemic justification for truth of his belief is not a defeasible one. However, we will see that both of these proposals encounter to Gettier problem.