You should base your PRIME range as per Enquiries column.
Something like...
Formula:
=Sheet1!$A$2:INDEX(Sheet1!$A:$A,COUNTA(Sheet1!$B:$B),)
Also, to ensure both PRIME & Enquiries has same dimension... Enquiries should be...
Formula:
=Sheet1!$B$2:INDEX(Sheet1!$B:$B,COUNTA(Sheet1!$B:$B),)
Also, for your COUNTIFS formula, to ensure that it works for any region setting, use Date() function instead of coercing "1Jan" into date.
Formula:
=COUNTIFS(PRIME,"PRIME",Enquiries,">"&DATE(2018,1,1))
This will return 28.
See attached.
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