The Russians won't permit Syria to utilise the S-300

Why do the Syrians appear reluctant to fire S-300 missiles at Israeli planes even when they enter Syrian airspace?

For the following reasons, Russia won't let Syria use the S-300:

Putin is not a fool, so increase. Even though he talks a nice game, he always takes great care to avoid escalating a conflict by engaging militarily with people or countries that could hurt him. Putin avoids firing on Israel's jets because he doesn't want to start a war. Think back to what Russia did after the US fired cruise missiles at Syrian targets, possibly resulting in the lives of Russian service members. He did nothing. Remember what Putin did after Turkey shot down a Russian plane? He did nothing.

Intelligence: By using a weapon system against a foe, especially one that uses radar tracking and targeting, one is essentially showing them how it works. The enemy will take note of and record all of the frequencies, search patterns, and other radar-related data, the ranges, the order in which you track and target, and the last step in which you close in on the target to attempt to shoot it down. The Israelis would also use a number of defences against the system while monitoring their effectiveness. NATO and the US would be given access to all of this information to help us build even stronger defences against it. For the same reason, the US does not. Krazy Kim's missile testsKrazy Because we don't want to show China, Russia, or North Korea how effective our anti-ballistic missile systems are, we do not destroy Kim's test missiles.

Reputation: For this reason, image is the key component. Russia praises these S-300 and S-400 systems as being "best in class" and adaptable. If these systems were tested against Israel and shown to be ineffectual, the Russians' reputation and the millions of dollars in revenue they expect from the sale of these weapons would be shattered. As long as you don't utilise them, the claims about how amazing they are won't be challenged.

These missiles are very expensive. Simply said, Russia cannot keep opening fire in response to each Israeli assault.