I am not sure I understand your problem. So, the array contains data for 10 videos. First 6 are shown on the screen. When you scroll with mouse wheel down, you want to show videos 2-7, when you scroll further down, show videos 3-8 and so on, is this correct?
I don't know why your code doesn't work, but you can do all this a bit easier. On mouse wheel down, remove first record from the array and insert it into the back of array. On mouse wheel Up, remove last record from the array and insert it into the front of array. This will allow you to simply display the first 6 records from the array. And you will not need loop1, loop2, loop3 variables, just use "Repeat 6 times" loop with loopindex expression.
Also, if you put ListItem, ThumbImage and Checkbox in the same container, you will only need to pick ListItem, the other two objects will be picked automatically.
And, if you are not doing this already, start using Debug Mode, it will allow you to monitor the array contents, all objects etc.