Can't boot in tri-boot laptop!?

At first, I have Ubuntu 11.04 in my laptop. After that, I installed Windows and it rewritten my MBR. So, I install the Ubuntu again in the other partition to take back my grub. This make my computer has 3 OS. After installing, I can't boot to the Ubuntu 11.04 when choosing the OS in the grub list, but I can boot into the other 2 OS. How can I boot into the Ubuntu 11.04?