The jewish leaders didn't see the miracles, they only heard about them. That's why they came out to see what all the hub bub about this man was about.
There were ordinary people that saw the miracles but all of them? Not likely. In a crowd of 5000 the people nearest Jesus would have seen but not the ones farther away. Even if the message that Jesus multiplied the fish and loaves was relayed to every person as they received the food, without seeing the miracles for themselves I imagine there were a lot of people that didn't believe the message. In most cases the individuals who came to Jesus for a miracle said they "heard" He could help. So many did believe based on what they had heard not what they had seen but not everyone was so easily convinced. (Besides miracles in themselves wouldn't be the proof that Jesus was the promised Messiah if the people were expecting a national redeemer and strictly speaking, Jesus was a personal redeemer OT scripture says jews and gentiles).
The jewish leaders were not going to believe in what they had heard, they wanted to see. However, Jesus wasn't there to do miracles for everyone, He was sent to redeem israel. Because His mission didn't give the religious leaders a role in Israel's redemption, He was working against their agenda. He had to be cautious too. If He had done miracles specifically for the leaders and then told them that they had no part in the plan He would have been killed sooner than He was. Remember, the leaders held great sway over the jewish population. Notably, people with a reputation in the community were sneaking around so they wouldn't get caught with Jesus and lose their place in the community. He was a big deal and the leaders knew it. They weren't going to believe hearsay, and if they saw the real deal, they were going to kill the miracle worker because they didn't fit into His plan. The only way to cause them to believe was pointing out in the scriptures that Jesus was the promised Messiah. In that way the decision was between the word of God that they said they ALREADY believed in and their own consciences. Jesus said, "if you TRULY believed moses, you would believe in Me also...".
So, it was about proving from the OT that Jesus was the Messiah sent from God not specifically miracles to prove that He was sent from God.
One obstacle to their belief that Jesus was the redeemer was the fact that His redemption was an eternal spiritual redemption and that wasn't something the leaders in particular thought that israel needed. They weren't sinners like the gentiles...they were in fine spiritual condition as far as they were concerned...they were superior to everyone else as far as they knew. Over time the leaders made up their own laws that superseded Gods law. Don't eat with gentiles, don't touch gentiles, gentiles weren't allowed to be too near the temple. All of these laws had erected a wall of separation and that was not what their "calling" demanded. They were called to be a light to the nations, not exclude the nations. Jesus talked about that too. He said that believers were to let their light shine before men so that men would see it and would glorify God. Not, hide the light like the leaders had done and convinced their followers to do. See, that's OT stuff fulfilled too..."the people that sit in darkness have seen a great light". They didn't see the light of God with any help from the leaders, did they?