Russia has several arms deals with Syria that it says it will complete delivery on despite the international arms embargo on Syria. Russia is not much concerned with offending the international community by breaking the embargo. Russia has done that before, although it tries to be discreet about it and denies everything, or comes up with some imaginative excuses for its actions. But there are other reasons to hold back on delivery. One is that these weapons will probably get their first combat test and quite possibly fail. This will hurt future sales of these new weapons.
Poor performance in combat has long been a problem with Russian weapons. During the Arab-Israeli wars, and the 1991 liberation of Kuwait, Russian weapons performed poorly. The Russians blamed it on operator error, pointing out that some Israeli generals had confirmed this by saying they could have let the Arabs use Western systems while the Israelis used Russian weapons and Israel would still have won. Despite this, Russia often had to give away many of its weapons or provide such generous credit terms that it was, in effect, giving the stuff away. After the Soviet Union collapsed in 1991, at about the same time a U.S. led force walked over the Russian equipped Iraqi Army and won decisively in less than a hundred hours, the demand for Russian weapons sharply declined. Many Russian defense manufacturers disappeared in the 1990s, and those that survived were the most capable and able, with some help from the bankrupt Russian government to survive by selling their best stuff (which was often pretty good) to whoever was able to pay cash. China and India loaded up on a lot of decent Russian military tech which, before, was not exported. Only “monkey models” (with the best electronics and other features removed) were exported. After 1991 you could have whatever you wanted if you could pay. Even the U.S. bought some of this stuff, in order to see just how well American weapons would do against the best the Soviets had.
In the last decade that “anything goes” policy has changed, and now Russia is demanding more, and getting it, for their best stuff. But this works, in part, because the Russians have been able to boast of snazzy new features without being contradicted by a disappointing reality. So the Russians are holding back on delivering S-300 anti-aircraft systems to Syria because they fear that if these weapons (similar but, according to Russian salesmen, superior to the American Patriot) would be defeated in combat by Israeli or NATO warplanes.
Other weapons are less of a problem, if only because they are less complex and less expensive. For example, over the last year Russia had delivered dozens of their high-speed P-800/Yakhont missiles. These have a range of about 300 kilometers and a 200 kg (440 pound) warhead. Israel responded with air attacks on Syrian trucks and warehouses containing some of these missiles. Israel also accelerated installation of its new Barak 8 anti-aircraft/anti-missile systems in their three 1,075 ton Saar 5 class corvettes (a prime target for the Yakhont). Israel has indicated that these attacks will continue, despite opposition from the United States. The worse scenario for Russia is that Syria or Hezbollah will use Yakhonts against the Israeli ships equipped with Barak 8 and lose. This will be great for Barak 8 but terrible for Yakhont sales.
Russia has also delivered 6 of 36 Yak-130s Syria ordered (because that’s all Syria has been able to pay for). That's not a big deal because the Yak-130 is basically a trainer aircraft which, like many jet trainers, is built to do double duty as a light bomber. The ten ton Yak-130 can carry an external load of three tons (of bombs, missiles, cannon pod, or fuel tanks). Max range, on internal fuel, is 2,000 kilometers. Against modern fighters the two-seat Yak-130 is toast but it should have no problem bombing civilians. There’s no danger to sales here.
Syria is also waiting on 12 MiG-29M2 fighters from Russia. This Cold War relic already has a bad reputation. Like many Russia warplanes, it looks great on the spec sheet but the reality is that many MiG-29s have been shot down by Western fighters and the aircraft is known to be expensive to maintain. Syria is not desperate to get these MiG-29s, if only because they know they will be spending a lot to provide aerial targets for Western or Israeli fighter pilots. What really matter are the four S-300 batteries Syria has ordered and partially paid for. Meanwhile, Russia is already sending (by ship and air freight) a lot of more mundane items of military gear (small arms, vehicles, communications) which no one expects to perform miracles. The S-300 is different and the Russians are really concerned about the S-300 being revealed as more snazzy salesmanship but not much help in keeping hostile warplanes out of Syrian airspace.