Why isn't Assad gaining back territory faster with Russian Air power there?

Because rebels also have TOW anti-tank weapon missiles that knocked out 20 of Assad's tanks the first day of battle in a "Tank Massacre".

Also, what many people haven't noticed is that Turkey, Saudi Arabia and other Sunni Governments started giving weapons to rebels of all sorts within the last 6 months or more and funding in addition to the TOW anti-Tank missiles given to some rebels by the U.S. to knock out Assad's tanks.

So, losing tanks is a really big deal for Assad because they are very expensive and tank crews (good ones) take a long time to train. And when a tank is knocked out by a Tow missile it is not uncommon that the whole tank crew dies. So, if 20 tank crews die like recently 3 to 5 guys in each tank likely died too. And Tank crews don't grow on trees that are battle seasoned like these crews likely were.

So, instead of a Tank Blitzkrieg it became a "Tank massacre" instead and these tanks and crews cannot easily be replaced monetarily or training wise. So, unless Putin puts in Russian Tanks with Russian Crews (which I don't think is very likely) you are likely going to see new Russian Tanks with Assad Crews who don't have a lot of experience on the battlefield.

So, getting bogged down on the ground is what happens when too many tanks are lost on a battlefield. Because you cannot push through small Arms fire fast enough without seasoned tanks on the ground. So, the ground war likely will get very bogged down for awhile on the ground and this likely will be very expensive (on all levels) for both Putin and Assad.

But, there is another factor here. Rebel Groups learned from IED makers in IRaq and Afghanistan how to make IEDs to blow up Humvees and tanks and armored personnel carriers. So, you have also got to know that IEDs (if they are big enough they can either blow up a tank or throw off a track from a tank which means the tank cannot move forward and only go in circles. So, this too, likely will slow forward movements also of Assad's and Iran's and Hezbollah armies pushing forward against Sunni Rebels and sometimes ISIS.