Array Formula which isn't populating further down the sheet

[Belucky]

I have attached a copy of what I am working on.

On the 'Project Stock' Sheet in column C there is the following formula: =IFERROR(INDEX('Do NOT Edit'!$H$2:$H$126, SMALL(IF('Project Stock'!$B$2='Do NOT Edit'!$G$2:$G$126,ROW('Do NOT Edit'!$H$2:$H$126)-1,""), ROW()-1)),"")

This formula works for the detail given in B2 - However when I change the detail in column B2 it does not populate anything in column C2.

I have also tried to add additional rows - B40 has a different product but it won't let me change the formula so it won't pull the information through from 'Do NOT Edit'

Has anyone have any idea how I can amend the formula so that when:
- B2 is edited it will populate the correct information
- Use the formula so that it can populate other cell information below B2.

Hope I have explained that correctly - First time using a forum