Almost everything about knowledge is contested, including the JTB itself. I shall try to signal where something is contested.
One reply is that your psychological feeling of certainty is no guide to the truth or otherwise of anything, and no guide to whether your justification is sound or not.
A qualification - Axioms and introspective knowledge are often regarded as exceptions, but not everyone would accept that they count as knowledge. The main ground for that is that you can't be wrong. Knowledge essentially involves risk (even deductive knowledge, though some people would strongly disagree - but mistakes are possible even there.)
Another qualification - Religious beliefs are another special case. People sometimes say they believe that God exists and sometimes that they know that God exists. I class such cases as special, and I'm certainly not including them in what follows.
Back to certainty. The JTB has the consequence that you cannot, on your own, validate your own beliefs as knowledge or not. ("I know that horse will win the race" says no more than "that horse will win the race" and expresses your conviction that it will. Nothing follows about whether you knew or not.)
The T clause in "JTB" means that a claim to knowledge needs to be validated by other people. Similarly, whether you are justified in your belief/knowledge also needs to be validated by other people.(There's a lot of debate about what standard of justification should be applied, but that's a different issue.)
That interprets your question as "If I'm certain about something, does it follow that I know it." But I think that what you are really asking is "If I'm uncertain about something, does it follow that I do not know it." The short answer is, no. But there is a complication. If you are uncertain about something, it may well follow that you do not believe it and so do not know it. So let's look at the relation between belief and certainty.
If you have conclusive evidence that p, you are entitled to be certain that p. (There is a lot of argument about what "conclusive" means in that sentence, but let's suppose it is possible.) Then you can say not only "I believe that p" but also "p is certain". But in using "believe" you are acknowledging the possibility you might be wrong, and so you are misleading me. You should say "I know that p", but not because you are certain of it, but because you have conclusive evidence.
If you have some evidence that p, you may well decide, that, on balance, p is most likely true. Then you can say (without misleading me) "I believe that p" and "on balance, p", not because you do not feel certain, but because the evidence available to you is not conclusive. If you were to say "I know that p", you would be misleading me. What if you turn out to be right? H'm. See below.
Now, quite often, we form beliefs on very scanty evidence, which is inadvisable in academic and legal pursuits, but in practice is sometimes necessary and sometimes merely convenient. In those cases, you would be right to say that your belief is little better than a guess, but you can say "I believe that p" and "p is uncertain". What if you turn out to be right? Again, see below.
Now, I think we are getting to the really tricky bit, and not everyone agrees (though I also think that a lot depends on the details of each example, and a global answer is unreliable) Suppose we ask the way to X, and our respondent replies with "Yes - I think you take the first left, no, the second left, and then right - or is it straight on - no, it's the first right ...." we would, rightly, be very reluctant to say that they knew the way. But suppose we finally extract a coherent set of instructions and, for lack of anything better, we follow them, and it turns out that the instructions were correct. There is good reason to say that they knew, after all. I qualify that because circumstances may altet the conclusion. If, for example, it turns out that this is their first visit to the area and they were purely guessing, of course they didn't know.)
Another kind of case is this one. Someone in a supermarket is answering customer queries "Where can I find x?", "What is the price of y?" and after ten minutes of this I ask you whether they know their stuff. They have hesitated quite often, so they seem a bit uncertain. But, assuming they give the right answer consistently, I maintain that you would be justified in saying they do.
Summary, with knowledge, getting it right is more important than being justified or being certain. To put it another way, in the JTB account, T trumps J and even (in some cases) an uncertain belief.
Philosophy can induce bewilderment. Which is fine, because the questions are often bewildering and that motivates the philosophy. But it is possible to carry on an ordinary life around it.
I'm sorry this is so long, but I couldn't think of a shorter way.