help needed debugging selection in "Cash register"

[speedone]

I'll try to keep it short: I've been busy working on a cash register. I'm a newby so it's probably full of things that I could have done different/better. I'm working on a sheet where I can specify sales by selection of month (Sheetname= ZoekenMaand).

Everything works fine for the months "januari" and "februari" (yes, it's in dutch ) but from "maart" it doesn't seem to work any more. I'm totally out of ideas why it doesn't work...

If anyone would be so kind to take a look at it and find what's wrong with my coding I would be very gratefull