One if these:
1. You are on a forked chain. The network will reach consensus that your submissions are invalid and reject what you send.
2. Your latency is too high, typically caused by mining over wifi or if there is something that is causing a delay with your network packets. It could be for example that you have a lot of noise or a last mile issue, causing that when you sumit a result that someone elses result is received and accepted by the network first and yours are rejected.
3. Some other technical mixup in your wallet.dat folder.
4. A low resource situation. You might be running other resource intensive applications on the same PC with your mining wallet. This could cause that the PC struggles with processing found blocks and submitting late.

Solutions:

1 and 3. Delete everything in the folder where your wallet.dat is except your wallet.dat and Loco.conf and re-sync your wallet.
2. Check your ping response with a utility like traceroute or speedtest.net and make sure it is low, at least <50, preferrably <20. If on wifi switch to fixed line or else have your line checked out by a technitian.
4. Close other apps and/or restart your PC with the mining wallet on.