I'm not sure without seeing the desc file or your pngs for the animation but some things to double check and consider...
if you open your desc let's say for example this is what is inside:
480 427 30
p 1 0 part0
p 1 0 part1
p 0 0 part2
the 480 427 is referring to the screen resolution size that will play on the screen
the 30 is referring to the frames per second rate and how fast the animation will move
the next three lines i have pasted above are showing part = p 1 0 part0 which is basically saying in laymans terms that part0 folder is going to play one time and not loop after the first time
same for part1
but part2 is going to loop and then stop when the phone boots
as for the files in those part0, part1 and part2 folders you must make sure that your image names are correct and in sequence I have found it easiest to use my screen size for labeling 480_427_00000 like this but if you do make sure you have 5 digits at the end for some reason it doesnt like it when i use less
the next image would be 480_427_00001, then 480_427_00002 and so on ...
when you are all finished make sure they are all pngs all in sequential named order and that your desc is correct. After verifying, when zipping you must use STORE method because compressing the images at all during the zip process will cause it to malfunction.
when you load it on the phone you MUST name is bootanimation.zip exactly like that or the phone will not recognize it as far as I have been able to tell.
hopefully this helps a little.
I have a thread with some custom animations in it if you wanna check it out you can download one of my animations and check out the structuring of my files and desc text if it might help.