Definitely call them directly 2+ weeks before arrival.

I requested a room via the Web site 4 months ago. Nothing but an automated response saying be patient and they'll get back to me.

2 months ago. Same.

1 month ago. Same.

This week for 10 days from now. "We're so sorry, we're full that night"

Given the Singapore reputation of efficiency and customer service, I was quite surprised at how this came out. I've never seen a hotel reservation system that works so poorly ( some India hotel chains come close). Perhaps because they are based around "blocks" instead of nights and can't use common booking services readily available.

Good Luck.