I have similar experience as yours. My son could only say few words when he was 21mths (媽, 爸, 波, 魚, wowo (dog)... less than 10 words).
However, my son suddenly learns and speaks much in the past few months. When he was 24mths, he can recognize and say all alphabets and the relevant vocabularies (eg. B - Bear, C - Cat, Y - YoYo, etc.) He can also recognize and say many animal's names, some are both in Chinese and English. Besides, he can name most of the relatives without reminding him.
We've been talking much to my son since he was a baby. My mother once worried that he had any speak problem. However, I found the main reason is my son couldn't sit long and pay attention to what we said. He likes to play around and explore the new things by himself rather than communicating with us. But when my son is older now, he seems having more interest in talking with people.
Recently, my son likes us to tell him what's this and what's that, and then he follows what we said. Sometimes, he names the things by himself when reading a books or taking out the toys from the bags.
Playing with children or going to school is also helpful. My son is now studying in nursery and he seems talking more although still most in BB language.
Anyway, you just keep talking and teaching your son, and try to use different methods to arouse his interest in speak.
My son was similar to your son. He just focus on his work/play and ignore us. He would come to us when he had needs (eg. hungry, boring). We had nothing to do at that time but just kept trying and trying.
My daughter was same as your case, she didn't really talk when she was 21 months, my family doctor referred her to speech specialist. I remmember that the speech specialist asked me how many sounds tha she can talk, and he added if she can said more than 10 sounds, it meants she is normal. Anyway, she really talk when she was 2.5 years old.
It is a really a headache to tell her to keep her mounth shut, and she is going to 5 soon.
My son was quite noisy when he was very young but he still speak late. It's difficult to tell now. Just keep talking to her and check the status regularly.