I've noticed that, when it's time for a scheduled (or unscheduled) songwriting session, my brain usually isn't in the right place. I've been working the day job, or maybe doing housework, or reading and writing email--that is, mostly using the logical side of the brain, rather than the creative side. And I can't just flip a switch and go from the logical side to the creative side in an instant. And sometimes, I just feel mentally and/or physically tired, so I'm subconsciously or consciously looking for excuses to not do the songwriting session.
I decided that the way to overcome that is to slowly phase from the logical (or tired) brain to the creative (and energetic) brain. Maybe I'll pick up the guitar and noodle around, maybe practice a song, play a cover song. Or practice piano. Maybe I'll grab a book of poetry and read some poems. But I won't start working on a song immediately. I have to first transition to the right frame of mind.
I used to think these things were just me procrastinating. I told myself, "you're wasting time, get to work!" But now I see them as necessary; like stretching exercises before you run or play a sport.