Invisible Chiffchaff

So this might sound strange, but I can not for the life of me actually see Chiff Chaffs when I'm out and about.

I can HEAR them but I can never see them. I've sat for a bit too in areas I can hear them but I still can't see where they're sat.

Is this a common thing with Chiffchaff or am I just not looking hard enough?!