The problem you will face is that phages are very host specific. So while there may be many many phages in your fecal sample, you won't see plaques unless you use the correct host as your plating cells. What you may wish to consider is whether you can find phages in your samples for some specific host, say E. coli (or something else of interest) and focus on those.

Michael J. Benedik In our research, we use E. coli and Salmonella as host organisms. Our objective is to isolate and purify lytic phages that specifically target these pathogens, which were isolated from the same fecal sample used for phage isolation. Unfortunately, no plaques were observed during the process.

Then it makes more sense if you are focusing on those two strains.

Assuming your plates look good and you can see plaques using control phage, then just try again with some different samples. You might also use some lab strains as controls, you might have had the misfortune to have isolated a host strain already resistant to the most common phages.

Michael J. Benedik This makes more sense now. I think we should procure control pathogens from ATCC and use them as hosts, which might help us identify the cause of the problem. Additionally, we plan to isolate phages from sewage first. If we successfully isolate phages that target our pathogens of interest, can we test their effectiveness against the pathogens we’ve isolated from poultry fecal samples? Or is it better if we isolate the phages from fecal samples? Thank you for answering.

It would have been better if you also started the isolation method by spot assay then proceed to double agar layer method