Trouble with combining MID and VLOOKUP

My supervisor assigned me a project where I essentially compare two lists. We want to develop a formula that will detect differences between two quotes.

Now this formula [=IF(ISNA(VLOOKUP("*"&MID(D21,2,8)&"*",Email,1,0)),"NOT FOUND","")] works perfectly on the first worksheet, but for some reason it doesn't want to work on the second sheet [=IF(ISNA(VLOOKUP("*"&MID(A7,1,8)&"*",Centre!B18:D45,4,0)),"NOT FOUND","")]. To make things even more perplexing, if I change the MID on the second formula to B7 and reference the product description rather than A7 and reference the SKU, then it starts working as desired.