not isNull in an if statement

If(
[Activity Type Code] = "4" and [Analysis Status] = "Complete", 120*[Actual Modeling Hours Required], 
[Analysis Status]="Complete" and not IsNull([Construction ROM / Scope Estimate]), [Construction ROM / Scope Estimate],
[Analysis Status]="Complete" and ([Cost/UoM]*[Capacity Regain])>0, ([Cost/UoM]*[Capacity Regain]), 
[Analysis Status]="Complete" and [Complexity Code]="1", 500, 
[Analysis Status]="Complete" and [Complexity Code]="2", 1000, 2000)

For some reason, the code does not reach the 4th line, i.e. it doesn't display the result ([Cost/UoM]*[Capacity Regain]). Instead it displays 0. But if I comment out the 3rd line which has the "not isNull" function, the code evaluates all the lines correctly.

The 3rd line works but for some reason the code doesn't get past the 3rd line.

Thoughts? Anything else I could try?