With so many periodical Israeli attacks against Syria, why isn't the Russian S-300 a game changer yet?

That 'gift' comes with restrictions from Mr. Putin.   Israeli missiles coming into Syria most often target Iranian positions, as they did on June 30 intending to hit IRGC bases inside Syria.    But Russia apparently wants to rule out every possibility of annoying Netanyahu with a response.   With Putin's political stance, Russian military hardware is rendered pretty much useless for Syria in defending herself against Israel.

Much too unfortunately, the Lebanese military as well as Hezb'Allah are prohibited by the international establishment to have an air force or an air defense system .. for the benefit of Israel of course.  Israel has been quick to take advantage of it, using South Lebanon's mountainous terrain around the Bekaa Valley as a convenient spot for launching missile attacks into Syria.  To deprive Israel of this illegal benefit, Syria would need to extend her air defense system up to Bekaa Valley or at least patrol the area.  Hezb'Allah would welcome that.  But Putin would not allow Syria to take such a step.  He does not want the international community to see any bit of Israeli interest thwarted with Russian weapons, particularly if the retaliation is carried out by a member of the Resistance.

It's been obvious for years that despite having a foot in both camps, Russia is adamant to steer clear of any official commitment to stand by Iran.  It's actually even worse than that.    Putin is discreetly focused on Israeli interest only.  Without a doubt, his devious foreign policy is helping Israel's planned killings of Iranian fighters on Syrian soil.  It has resulted in the deaths of scores of Iranians since Russian presence in Syria. 

Connect the dots.  Analyze the recent bluff organized in occupied Jerusalem - the National Security Summit on June 25 with Russia as one of the participants along with Israel and U.S. .. .. and there was some duplicity from Moscow pretending to defend Iranian presence in Syria.  Five days later Israel carried out yet another missile attack targeting Iranians stationed in Syria.  Moscow did nothing.  The duplicity was revealed inadvertently by Bibi himself in his closing statement at the end of the Summit,  "all of us (including Russia) agree on the end goal of getting Iran out of Syria."   This was preceded by the statement from the Russian security & intelligence officer during the Summit on "ensuring Israel's security."    

A month ago, the Russian president turned down Iran's request for purchasing the advanced S-400 missile defense system.

This dirty three-some of the National Security Summit isn't fooling anyone, if that's what those buggers are presuming.