This is an old, but annoying bug. Often when you click a collection item the checkbox in the achievements doesnt fill in. Its really there but it just doesnt look like its done in the window. The fix is to open another year's achievements then come back to the one you wanted to see. Its been like this forever. Please fix.
This is highly dependent on the lag you are getting. Last night I was filling collections on my paladin and had zero issues with each individual checkmark showing up by 5 seconds after click. For an entire expansion's worth of collections. Other times, it's only been possible after clicking checkboxes multiple times. The developers know about lag. This is lag. There's nothing special about collections (or any other achievement for that matter). You are asking them to fix lag.
