Thanks for your reply.

Actually the mother bulb wasn't small, it was good but I was late at planting it so it didn't have time before winter came.

How should I know which bulb growing now is the mother bulb?
I shouldn't separate the bulbs now after they started growing, right? And if I let them be in the same pot, will the mother bulb (or any bulb) be able to flower? I really want to see some flowers this year!

You can repot the whole set in a new pot with new soil. The mother bulb will be bigger in size than the bulblets. Usually the one in the middle is the mother bulb surrounded by small bulblets. You can definitely separate them and pot them separately for them to grow. It really won't make difference to the flowers but if you separate them the mother bulb might put all its energy in making new bulblets again. Wait this year to see if it blooms. If not start with new sets.

That's strange, I see no bulb in the middle, only the new bulbs surrounding an empty space.

Yes, I will leave them together this year and see. I hope at least one of them will flower.

Some tips I've found online- full sun, fertilize, make sure your potting mix is well draining, don't crowd the bulbs in a pot (something I tend to do and I hardly got any blooms).

http://forums.gardenweb.com/discussions/1911587/tuberose-havent-bloomed

Another tip from the book The essence of paradise by Tovah Martin is mixing compost into the potting mix.

Thanks for the tips Robert. I usually do them, and I only put one bulb in the pot because I don't want to crowd them. But my problem is one bulb produces many green sprouts, and they are so crowded (I explained that above).