It's not fluid because there's no path of movement, no continuity. What you get then is a lot of jitters or inconsistent-looking framerates (from held frames).
The head of the sprite has a motion path, up/down (but has held frames) but the fire looks blocked in without structure or hierarchy. You basically winged it. It looks OK in individual frames, but you put it together and it spazzes out. Making it fluid is what animation's all about.
Check out
this post about animating fire, study these examples.
Block out motion and all that with a large brush, do a hell of a lot of frame flipping to check if the base motion is correct before moving on to details.