Cascading Date Prompts not working

[Revelator80]

I have prompts running from my time dimension for financial year and month (to get YTD and MTD figures). When I try to cascade from the year down to month, it sometimes greys out the month box and disables it (although this doesn't happen all the time, it does happen a lot of the time). If I don't cascade then I get repetition (e.g. April, April, April... for all the months). Any idea how I can get around this?