If you have any IO issue ,then in RDBMS alertlog file ,you will see read/write failure messages as rdbms level IO capable processes do IOs.
If there is any disk issue ,it will get reported on ASM alertlog file .
So,at the time of delay do you see anything at ASM level alertlog file,if not ASM is working fine .
Delay in sending report could be possible because of few issues,
+ Slowness at EM to refresh
+ Mail server issue
+ Network level issue .
Thanks for your valuable response and time.
Looks like some of the data is too old and not sure my tech lead told me that he re-built some indexes and now the report is reaching us in time. Since I don't have access to the grid I could not gather much info.
And one more info that the report is collected from the table called ASM_USAGE_REPORTS