There are multiple reasons as to why not every plant and animal turns into a fossil. The first thing that must be considered is the type of organism. Some organisms have only soft body parts that have a lower chance of being preserved because they simply decay too quickly, while others have hard body parts such as shells, bones or teeth that have a greater chance of being preserved.
In general, for an organism to become fossilized, it needs to be buried rapidly in order to be protected from the elements and predators. This can happen by being covered in sediment in areas of high sedimentation such as the bottom of a lake. It could happen by being trapped in amber or volcanic ash as well. Because of this, fossilization often depends on the environment. Organisms that die in areas where they will be exposed for a long period of time have a much less likely chance of being fossilized.
Because it is fairly rare for an organism to become a fossil, our fossil record is not a complete record of life on earth. It is biased toward organisms that have hard body parts and those that lived in areas more favorable for fossilization.
I hope this helps and happy studying!