This means the fee will be different on most transactions. You can view the current network fee recommendations at https://etherscan.io/gastracker where they also display the average cost in fees for common transactions made.
You can try to use a 'high' fee in the app, which will take network traffic into consideration when calculating the fee price. If the network is very congested and continues to fail however, you might care to set a 'custom' transaction fee, and set it even higher than 'high'.
(Use the gastracker link above as a reference for current gas prices.)
Tap on the fee field when creating the transaction, and the options for 'Low', 'Optimal', 'High', and 'Custom'.
If your transaction is currently still pending, you will have an option to 'Accelerate' the transaction. This will double the fee previously submitted to the blockchain. You will see this option by going to your Ethereum wallet, locating the pending transaction, tapping on it, and the option to accelerate will appear on the menu that shows up there.