rsync with remote fat32

I've a script that syncs files between my server to my mobile via sshpass:

sshpass -p password rsync -vrz source1 source2 -e ssh user@ip:dest

using

-vrz 

rsync everytime checks all files to mobile,

using

-vrz --update --modify-window=1

rsync doesn't update newer files from server to mobile

how are the correct options to use rsync with fat32?