If you have doubts about the calendar, create a new one called e.g. "test" and create in this new calendar a new event filled with anything with an alarm for 30 days before.
If the alarm window shows, then I would say to move one by one each event from the old calendar to the new (but this won't do anything) or recreate the events with trouble.
If the alarm doesn't show submit here the calendar file with the one event. But again, I don't think that here's the problem.

Are the events old? I mean, did you dismiss or snooze the event at least once?

Or search in the user folder (FAQ) for the .ics file, open it with a text editor, search for the problematic event (BEGIN:VEVENT ... END:VEVENT) and paste here ONLY this event (CHANGE ANY SUBJECT OR DESCRIPTION IF PRIVATE BEFORE). This will show if there's something wrong with the event data.

Here are the two events I complained about earlier:
I created them a while ago and they didn't ever trigger (i never pressed snooze or anything)

BEGIN:VEVENT
DTSTAMP:20120529T232734Z
TRANSP:OPAQUE
UID:{61afbe44-616f-2740-985b-40035efa2d80}
SUMMARY:renamed text 1
CLASS:PRIVATE
LAST-MODIFIED:20120529T233136Z
DTSTART:20120606T210000Z
BEGIN:VALARM
ACTION:AUDIO
TRIGGER;VALUE=DURATION:-P4W
END:VALARM
END:VEVENT


BEGIN:VEVENT
DTSTAMP:20120529T233738Z
TRANSP:OPAQUE
UID:{8aff71c6-df0d-f84a-a225-13000d3d0fde}
SUMMARY:renamed text 2
CLASS:PRIVATE
LAST-MODIFIED:20120603T205046Z
DTSTART:20120606T200000Z
BEGIN:VALARM
ACTION:AUDIO
TRIGGER;VALUE=DURATION:-P15D
END:VALARM
END:VEVENT


Also... I did another test this morning after trying your last email advice and created two events:
one triggered, the other didn't:


BEGIN:VEVENT
UID:{eecc4e5e-b5b8-284a-bdb4-6c71ef28e2cd}
SUMMARY:Test
CLASS:PRIVATE
DTSTAMP:20120603T203515Z
DTSTART:20120629T210000Z
TRANSP:OPAQUE
X-RAINLENDAR-LASTALARMACK:20120603T204916Z
X-MOZ-LASTACK:20120603T204916Z
LAST-MODIFIED:20120603T204916Z
BEGIN:VALARM
ACTION:AUDIO
TRIGGER;VALUE=DURATION:-P30D
X-RAINLENDAR-ALARM-TYPE:alert
END:VALARM
END:VEVENT



BEGIN:VEVENT
UID:{f095ccd4-52db-cb47-a99d-3d85474c6644}
SUMMARY:Test1
CLASS:PRIVATE
DTSTAMP:20120603T204941Z
DTSTART:20120628T210000Z
TRANSP:OPAQUE
X-RAINLENDAR-LASTALARMACK:20120603T204945Z
X-MOZ-LASTACK:20120603T204945Z
LAST-MODIFIED:20120603T204945Z
BEGIN:VALARM
ACTION:AUDIO
TRIGGER;VALUE=DURATION:-P30D
END:VALARM
END:VEVENT


I am just about to make 3 events - test3 , test4 , test5 with different notice periods. Again, none of them have been touched apart from initial creation. I'll also backup rainlendar immediately just for evidence

Ok, as promised, created 3 new events test3 , 4 , 5. They are below.
It's been about 10 minutes and none of them triggered.
I have screenshots of the events 'edit' if you want, and below is the VEVENT for the three.


BEGIN:VEVENT
UID:{b515cd3a-eaae-5b45-be02-56b79a74e541}
SUMMARY:test3
CLASS:PRIVATE
DTSTAMP:20120603T231515Z
LAST-MODIFIED:20120603T231515Z
DTSTART:20120701T233000Z
TRANSP:OPAQUE
BEGIN:VALARM
ACTION:AUDIO
TRIGGER;VALUE=DURATION:-P45D
END:VALARM
END:VEVENT
BEGIN:VEVENT
UID:{e67f5359-2459-a746-bbb8-090f7e4c92d0}
SUMMARY:test4
CLASS:PRIVATE
DTSTAMP:20120603T231533Z
LAST-MODIFIED:20120603T231533Z
DTSTART:20120703T233000Z
TRANSP:OPAQUE
BEGIN:VALARM
ACTION:AUDIO
TRIGGER;VALUE=DURATION:-P60D
END:VALARM
END:VEVENT
BEGIN:VEVENT
UID:{9fcadf46-6e32-814a-bbc5-b2cb2e058810}
SUMMARY:test5
CLASS:PRIVATE
DTSTAMP:20120603T231551Z
LAST-MODIFIED:20120603T231551Z
DTSTART:20120704T233000Z
TRANSP:OPAQUE
BEGIN:VALARM
ACTION:AUDIO
TRIGGER;VALUE=DURATION:-P75D
END:VALARM
END:VEVENT
END:VCALENDAR

anything else I can do?
Thanks.

Oh well... I finally found (something) where's the problem and it's a mix between your events and Rainlendar config values.

Right click on a window -> Options... -> Advanced -> Alarm (is the last section) -> Hide from alarm after days

By default, this value is 7 days. test3, 4 and 5 alarm are more days away than this. Same as renamed text 1 and 2.

Edit: Also, you can remove the lines with X-RAINLENDAR-LASTALARMACK and X-MOZ-LASTACK. This will erase all info about previous alarms showed.

Aha!!!
Problem solved, now alarms that 'quietly' disappeared from the calendar are showing up. I took it back 100 days and a few events that timed out past 7 days quietly dropped off the notifier, meaning I didn't know about those tasks after they disappeared.

We'll it's also part my fault because I am using the event as a time driven task, and if I don't do it on time I click snooze for a few days and so on.

But now by putting 100 days there I effective have 3 months worth of 'snooze', right? Also solving, for example events that are 10 days ahead, with 25 days notice, 25-10=15, so 15>7, therefore no alert.

Ken.