Custom entities not spawning when behavioural characteristics set to Mob

Hi all, 

I know variations of this have been covered previously but I wasn't able to find anything that was an exact match to the problem I seem to be experiencing. 

I've added in several entities to the game which are either passive until attacked, or aggressive towards the player, however when I try to set the entities' behavioural characteristics to 'Mob' in the behaviour tab, the mob is no longer able to spawn in properly using spawn eggs. When I use the spawn egg, the mob will spawn for a fraction of a second and then immediately despawn. I have tried changing the characteristics to creature, however this seems to prevent them from being aggressive towards the player (despite any AI and Goals conditions), and I have also checked that the natural spawning type is set to 'Monster', however am still encountering the same issue. 

Any ideas on how to fix?