Just compare" was supposed to do" with " should have done ", what is the difference？
They both seem like " I needed to do this thing but I didn't do it actually."

Someone told me "was supposed to do" is like " I was told to do something that I don't think I need to, then I didn't do it.'' and " should have done" is like " I also believe I need to do it , but then I didn't do it . ''

However, another one told me "was supposed to do" is more focus on " The thing you didn't do is not a big deal actually " and ''should have done" is like " You boss told you that you have to finish this task, which really matters, but you didn't do it "

I am confused.

Question

Just compare" was supposed to do" with " should have done ", what is the difference？
They both seem like " I needed to do this thing but I didn't do it actually."

Someone told me "was supposed to do" is like " I was told to do something that I don't think I need to, then I didn't do it.'' and " should have done" is like " I also believe I need to do it , but then I didn't do it . ''

However, another one told me "was supposed to do" is more focus on " The thing you didn't do is not a big deal actually " and  ''should have done" is like " You boss told you that you have to finish this task, which really matters, but you didn't do it "

I am confused.

Answer

a) "supposed to have + past participle" and b) "should have + past participle" are very close in meaning.

b) is more general and so is more important to use well.  Someone has failed or failed to fulfil an obligation.

e.g. I should have cleaned the house before the guests came for dinner.

a) is more specific. It implies an arrangement, or that something was agreed or scheduled, and someone failed to fulfil the agreed / arranged obligation. The obligation could be a weak or strong one and other context would need to make this clear.

e.g. I was supposed to have cleaned the house before my dinner. I promised my wife I would do it.

1) She will be so angry with me!
2) It's no big deal - I can do it another time.

"should have +pp" would be OK for my second example too.