Your link was nonfunctional for me.

It sounds like some days you get a cumulative figure, and sometimes it comes through as zero. Add a column that uses either the prior cumulative number or the number above. E.g.

A2=15
A3=18
A4=42
A5=55
a6=0
a7=0
a8=0
a9=80

Then E1=0
E2=if(A2>0,A2,E1)
F2=E2-E1
And copy down.