If a dip in form can’t be attributed to something external, and seems mysterious, I think it’s usually a signal to try something different. Annoyingly, winning techniques don’t last.
You can think about changing:
Your sources of ideas: If you look to newspaper articles, think about mining your own experiences. If you need an emotional spark such as anger, try a different flavour. If you work it out with a slide rule, try picking something that seems counter-intuitive.
Your structure: Try telling the story backwards. Try drawing a shape and seeing what sort of plot it suggests. Try a different point of view character than the obvious.
Your working patterns: If you’re typically a marathon writer, try sprinting. If the words come slowly and painfully, try blurting it all out knowing you’ll do one additional rewrite. If you write straight onto the computer, try a pencil. Draw some of it. Write scenes out of sequence, and if the connecting scenes seem boring, don’t do them. Leave yourself a fun scene to the end, as a treat.
Try not to: succumb to panic and/or superstition (‘Sh*t! The cat’s kicked my mascot into the litter tray! That’s why this draft is rubbish.’). Try not to use this as an excuse to divorce your spouse, sack your agent, have a mid-life crisis, blame the BBC, emigrate, etc.
Don’t panic. Stay fresh.