I don't know how the adverts get updated by when they are completed or not (guess the seller does that after a period of time) but often I have found that sellers list the item on multiple places. When this happens if they sell it, they're often unlikely to update that places they didn't sell it through unless they just happen to go back to that site. If you are contacting sellers about items that haven't recently had activity, then it's probably sold and they don't check here often enough. Or you're contacting sellers about items that have completed and they can't be bothered to reply, whether they have seen your message or not, as they have marked the item as complete.
Also, life happens, work etc so this could be why people aren't replying as soon as you'd like.
If I were you, I'd make sure you're contacting only about items that have been listed or bumped recently, make sure your messages are polite and you'll see a good response rate.
If you contact me about a completed advert, something from a long time ago or with a message along the lines of "I'll give you 50% of the advertised sale price" - I'd just ignore you.