[Q] stuck at google logo after flashing kernel topic

my nexus 5 is freshly rooted with twerp my question is when I flash cm and try to install a kernel it gets stuck at the google logo with the unlock icon at the bottom is there a reason for this I install rom then kernel and then the gapps am I doing this wrong? or is there something im missing I have searched the forum and google sorry if im asking the same question over thanks