Problem with LOOKUP formula

I have a simple LOOKUP formula which is 

=LOOKUP(B77,'Members List'!$A:$A,'Members List'!$B:$B)

It is working in some cells, but not in others.

Per attached - Cells C50 - C58 have exactly the same formula, except they each reference the corresponding entry in column B.

Cells C62 - C65 have the same formula as C50-C58, with each reference changing as per, however results show as #N/A

Cells C69 -C87 have the same formula and is looking for results from B69-B87, many of which are the same as those in B50-B58. 

Why is the formula working in some cells, and not others. I am completely stuck and cannot see what the problem is.

To add to the confusion, if i add the value from B62 into B55, I get the correct result displayed. Same with any value from B62 - B65

Any help would be appreciated