If you send more than 20 requests on one day, the rest rolls over to the following day. If you need a different part the following day, you have to make sure you send a new request exactly at the daily reset time. The new request will then overwrite the one from the previous day.
If you send the new request, lets say 5 hours after the daily reset time, chances are that your neighbours have already accepted the request from the previous day (potentially for the part you no longer need).
You may be thinking that your new requests are not being accepted, while in fact you may have already received the unwanded parts from the previous day and not paid attention to the increased number of the unwanted item in your gift folder.
That's what I found some time ago was happening for me anyway. Luckily the reset time for me falls at 8am, so it's really handy as it's just as I'm getting ready for work