Why is solder not sticking to the PCB pads after using the hot air gun?
I was mimicking what I seen in youtube videos when it comes to
  replacing SMD components. I add solder to pad since this uses the flux
  I then add-flux grab the part heat the solder lay part down then it
  looks ok and then after it cools the part and solder slide of the pad
  it loses it's weld and i don't know why


I don't know what videos you saw, but it seems you are doing everything backwards.

You don't need flux if you are using solder paste, but if you want to use it you apply it either before solder paste (to prepare pads) or after soldering (to repair bridges).

You don't heat solder before placing the part. First you apply solder paste to pads, then place the part into cold paste (you can warm-up component before placing, but it is not necessary). Then you heat the board and component evenly and gradually. When paste changes its appearance (releases flux) you bring heat gun closer and wait till solder melts and part pins "sink" into it. Then remove heat gradually but without delay.