The Concept Of “Maturity”

Both masternode rewards and staking inputs are subject to "maturity". Effectively this means that rewards earned from masternodes are not withdrawable for a period of 'x' blocks. For staking, if you stake an amount (or receive a reward on a given staked amount), then the whole amount becomes "immature" and is therefore not withdrawable for a certain period. It's further worth noting, that the staking input will also only be eligible for more rewards after 'y' confirmations.

With regards to staking.. remember now, that a reward received is not actually a separate transaction, which you receive. This can be illustrated with a simple example.. Say you stake 1000 coins of a particular crypto and then receive a reward of 10 coins, the 1000 coins is considered a staking input of 1010.. It it stakes again, then all will become a staking input of 1020 etc..

In a nutshell, the "immature" crypto needs to "re-mature". This may make withdrawing sometimes appear slightly troublesome. Waiting a while after a reward and/or trying to withdraw in smaller increments, may indeed help with any such difficulties.