ok just noticed that สมาชิก sa L maa M chik H is pronounced as this - สะ-มา-ชิก where as i assumed it would be like สะ-หมา-ชิก

So why does the tone not change ? I am guessing be cause its a tri-syllable word ? เอาช่วยด้วย!

Looks like it's an irregularity

Thai System of Writing (Mary Haas) page 52 and 53

7. Exceptions in the Treatment of Consonant Groupings

It may be stated as a general rule that an initial HIGH or MIDDLE consonant will govern the tone of a following LOW sonorant when no written vowel intervenes but this rule is not an invariable one. In some cases a following LOW sonorant will remain LOW . . . couple examples

สมาคม club society

สมาชิก member (of a club)

I agree with Aitch above: it appears that the สมา- words don't follow the tone-shift rule.

In my copy of So Sethaputra's dictionary, he has สมา (apology), สมาคม (to associate; an association), สมาจาร (right behaviour), สมาชิก (member), สมาทาน (acceptance of the Buddhist commandments), and สมาธิ (concentration); none of them have a tone shift. Interestingly, สมา is defined as being the same as ขมา (apology), which also doesn't have a tone shift.

Some other common words which look like they ought to have a tone shift, but don't, are สภา (assembly, council, parliament) and สภาพ/สภาวะ (state/condition).

thanks!