I suggest to have a look at following script which is provided by oracle to monitor IO.
ASMIOSTAT Script to collect iostats for ASM disks [ID 437996.1]
Further this i have questions
1) Are you logged in asm sysasm? Make sure that you have proper rights
2) Did you check the liveness of asm instance? Make sure its up and functional also check the asm alert.log for awareness.
Hope this helps