Hi Will31,
I think the only issue is "=>" versus ">="
Try moving the = sign after the > sign. It's working on my sample file I made.
=SUMIFS(Data!$G$2:$G$3000,Data!$A$2:$A$3000,CTS!$A40,Data!$F$2:$F$3000,">="&CTS!I$2,Data!$F$2:$F$3000,"<"&CTS!J$2)
Cheers,
Kevin