Many “true bypass” pedals, or long bad quality cables can create problems such as dull sound, loss of tone …
In this case, you can try a buffer, or even two buffers : one at the beginning of the chain of pedals, one at the end. Listen and decide what sounds best to your ears !
Please note that Fuzz effects and vintage-style treble booster may sound awful when placed after a buffer !
In this video, I use a “bad cable simulator” I built for show how a buffer can help to deal with long cables and/or many true bypass pedals :
For more information, see : http://www.it11audio.com/simple-buffer/