To simplify the spreadsheet, I have Users listed across Row1 from B to G; and
Activities listed down ColumnA. Each User has to enter their time against an
Activity. The total time allowed for all Users per Activity is shown in
ColumnH.

I selected the entire Range and entered under Validation Custom Settings
this formula: =SUM($B$2:$G$2)<=$H$2 and clicked "Apply these changes to all
other cells..." and clicked "ignore bank".

The Validation works in the first row but does not work in any of the rows
below.
As usual any help in my shortcomings is greatly appreciated.