Sounds like they are gobbling up all the available N during their growth spurt thru the stretch and coming up short after. They use a lot of N during the stretch and with organics it takes time for the bio-herd to reduce the organic components into the salts the plants need to feed on.
You might want to try adding the guano when you begin flowering so it has time to be made available when needed.
Soil temps are important as well. If too cool the bacteria and fungi slow down and that reduces available nutrients. Do you add any beneficials to your soil during the grow or sweeteners to support their growth?
I use soilless and hydro nutes myself but have read enough about organics in the last 40 years to know there are a lot of biological factors going on under the surface that need tending to so you get the best results.
I found a great spot to download
FREE POT BOOKS. I downloaded a grow bible first and got lots more. Books look great and complete like the real ones I have here. No web site but just a page of links. Just right click on what you want and then "Save Link As" to download so they don't open first as some are 50+ megs. They got lots. Enjoy.