Why don't I see fluorescence signal of phalloidin conjugates in my paraffin fixed tissue?

This can occur for a number of reasons. If you fail to get good staining with phalloidin then you may have lost the cytoskeleton ultrastructure at the fixation step. You can verify your staining method on some cell culture cells to be sure your phalloidin reagents are good. You may then try tissues with different fixation protocols to make sure the tissues are fixed and cell structure is stabilized. 

The other possibility is that when cells and tissues are treated with solvents such as xylene or acetone (for example during deparaffinization of tissue sections), it prevents phalloidins from binding to F-actin. Frozen tissues, which are not typically washed with organic solvents, can be used instead of paraffin fixed ones. Another way is to use anti-actin antibodies.