It would be nice for Assad if a sudden increase in regime progress or an unexpeted decrease in rebel abilities or motivation were th explanation. There were two prime causes: Shortage of ammo and failure of other rebel units to support. In that situation thousands of rebels did the sensible thing and withdrew.
The bad news for Assad is that such opporunites have been rare. In one case, deception also played a part but hasn't worked since but ever since infiltrators have tried posing as rebels and have been caught, tried and executed. Rebels have not forgotten that the one and only success infiltration was followed by a brutal massacre of defenders who surrendered shown on a video where regime thugs kicked heads and stood on bodies and smiled.
In the Damascus area, FSA units working under General Idriss are much better coordinated and thus more likely to support one another Long-pent up weapons and ammo are likely to pour in from Jordan as the Congressional vote on bombing Syria approaches. Releasing those weapons to moderates was part of a between Obama and top Republicans.
It is one thing to repeatedly break promises to the moderate rebels and quite another to break a deal with the GOP because of certain political consequences. Not only could it cost Obama the vote he needs on bombing, but such a betrayal would turn him into a total lame duck. Obama has enough problems with Republican obstructionism as it is. I Thus the days of withholding weapons to moderates may be gone forever.
Putin: "RUSSIA MAY SUPORT OBAMA'S BOMBING OF SYRIA IF...
...Putin can be convinced that the regime ordered it beause doing so would demonstrate outright and insupportable craziness.
Obviously that "if" which leaves the door open either way but why say it all? Surely it will make regime insiders in Damascus nervous as they ponder that question.
Putin may be in denial about how many ways Al Queda and especially Caucausus extremists continue to benefit the longer Assad stays afloat (Chechens even formed a new brigade yesterday) but he can't deny the extremely high probability that as the regime shifts around chemical weapons the extremists must inevitably succeed in taking a base or seizing a convoy with such weapons. If they do, you can bet on three things.
1. Neither Putin, nor Khamenei nor the West will be informed by the embarassed regime.
2. The captured chemical weapons will be moved immediate to hiding--almost cetainly to more than one location.
3. Though Al Queda has many targets it would prefer to hit, those most likely to be targeted first are those were proximity and access are borders are most convenient--the Assad regime, Lebanon, Iran, Russia and pro-Russian governments in the Caucausus.
The latter two are especailly near and dear to the hearts of Chechen fighters in Syria who have vowed to head off for Russia as soon as Assad falls. For Putin, it's better that the do so now than later.