I have had some experience with this and your solder paste is probably old.
It appears to dry out, but that is not exactly what is happening. The solder is made up of tiny little spheres of solder with, an incredible amount of surface area. This mean that there is a lot of reaction with the solder and the flux that it is mixed with (refrigeration can slow this down some maybe 50% more life, 4 months?). Even if you get the solder back to the correct consistency using a nonvolatile liquid such as liquid solder flux, it will still have too much oxide in it that will cause the solder to not work correctly. The solder flux carries the oxide under the components and can cause some resistances around your component connections. This will cause all kinds of problems especially if your circuits are high frequency.
This is an aggravating problem and the best solution is a good solder paste that is from a volume dealer. I recommend just buying what you need. If you are going to use a guide template then use solder in a small jar and use an old credit card to spread it (then using a reflow oven is great, I use a Black n Decker toaster oven with good success).
If you are hand soldering an SMD component here or there, then you should use the syringe a 1ml syringe is best but use a 5 or 10ml syringe to load it. If you have a steady hand then you are in and if you have a microscope even better. First put the solder paste on the pads and then use an needle or dental pick to carefully move excess on to the pad or remove it. Place the part using tweezers or vacuum pick tool. Then with a needle hold the part in place, touch the tip of the soldering iron to the edge of the pad on the board that you are soldering. The solder will flow quickly and right on to the component then do the other side of the component or opposite corner of the component if multi-pin. I would not use dry solder on SMD components because it is too slow. Remember if you are using solder paste, it will already be under the component connection and your solder is already in place so you have a free hand. Imagine hold the part, the solder and the iron...
All of the information here is from experience. I even tried thinning with IPA.
On some safety notes soldering with SMD solder paste especially old solder paste creates a lot of very toxic fumes, much more than regular soldering. Good ventilation and a fan are necessary.
Solder paste with IPA in it can cause lead to pop into your eye.
And last, this stuff would be deadly if it were ingested by a child (or an adult for that matter) Hands dirty with this stuff will get on your sandwich too! so wash up often.
The answer is get new solder paste. I thought that my whys were useful.