There are a few reasons and the most common in not-yet-synced client is if at least one of the input of the transaction is invalid.
You can also try to manually broadcast it and see what mempool-related error you'll get:
- In the transactions tab, right-click on one of the transactions, then select: "Copy raw transaction"
- Open the console (Window->Console), then type: sendrawtransaction "Your_Copied_RAW_Transaction"
- And it should return with an error which should be the reason why those aren't successfully sent.
If it somehow proceed, then there could be in your setting that's preventing txn broadcast..