As others have mentioned, you need to increase the transaction fees to an amount that you would be sure that the miners will include your transaction in the next block to be mined.

How much fee you should pay changes every block, and an easy way to make sure that the fee is enough is to use a wallet that calculate this fee for you, I used Electrum in the past, and it did a great job at calculating the fees which made all of my transactions appear just in the next block.

Hope this helps.