Formaldehyde/formalin is not a good fixative for nucleic acid preservation. It reticulates proteins, making the histone-packed DNA difficult to extract. Moreover, there is water that hydrolyses nucleic acids with time.
You should use material fixed with Methanol, Acetone, Ethanol, or Ethanol:Acetic 3:1instead, and keep it in a freezer.
If you have only FAA fixed samples, you can try to add proteinase K in your extraction buffer.
Olivier Catrice thanks for your kind response and detailed information.
I have extracted the DNA in January 2020, but the quality of that DNA is not very good. At this time, the only available source to extract DNA is the floral samples that are fixed in FAA.