First time laughing - HELP PLEASE

Becky

Hi all, this is my second baby, my first being a gorgeous little boy who turn 4 on Thursday who is also autistic. Now he has few words but communicates with us just fine, laughs, cuddles, kisses and looks us on the eye etc. He is very kind and loving. Now my second child is my little girl who is 15 weeks old and has given us loads of brilliant eye contact and huge smiles but we cannot get her to laugh for us, I remember my son being the same, he laughed for the first time maybe 2 years old, we never got the old belly laugh from him as a baby and now I'm worried that I can't make my daughter laugh either, what am I doing wrong?! We've tried everything, songs silly faces tickling peek a boo but we get gorgeous big smiles but no giggle, chuckle or belly laugh. Obviously my imagine is running wild about why this might be and I guess I just wondered if anyone had any tips/advice/wisdom for me.

TIA

Becky