First of all, they are called "straps" not FSB holes. Just like your ram timings have to be either loosened or more volts added as you increase Mhz, your NB has timings that change as the Mhz increase. Try jumping 100Mhz on your FSB and try again.
Let me give you an example as how this works. I had a 680i board that had a 1066 strap that end at 1300. At 1301 the 1333 strap started. When I got to around 1220-1250 I really had to start increasing the NB voltage. At 1270 I had to start giving SB voltage. I could take it all the way to 1300, but I had to max out both the NB and SB voltage. When I reached 1301 I could bring both the NB and SB back to default voltage because the timings changed to looser values. With this being said you my wonder about performance. When just like ram, your board performs a little better with tight timings. I got a 80MB/s mem bus increase at 1300 vs 1301.
Sorry for the rant, but I use to believe in FSB holes too. Though, of course there may be boards out there that with max NB and SB voltages they can't even reach the top of a strap and have a hole where it is impossible to post. Haven't played with any yet.