The most common cause of failed payments is setting too low of a fee limit.  Some Lightning wallets require higher fees or minimum amounts to be sent.  During the Send flow, the fee amount can be set to a custom value.  The fee amount is not necessarily the amount that will be paid, but instead is the maximum amount that could be paid for fees.  Lightspark strives to send over low-cost routes but will attempt to use higher-cost routes if needed and the fee limit accommodates the higher fee.