Porous, organic materials like wood, leaves, fabric and even some soft, lower quality papers contain air and moisture, which they absorb and emit depending on the climate. These materials will continue to "breathe" ... releasing air even after being covered with resin. This is called off-gassing and it results in air bubbles in the resin, sometimes hours after you have poured and torched.
To help prevent this, you need to pre-seal your work, prior to resining.
You may still get a bubble or two ( in which case you will need to sand down your piece and apply another coat ) but pre-sealing can greatly help to minimize bubbles.