It's a great mercy to not be able to remember the entirety of a past life, as Cryscat very expertly put.
I have had four, that I know of, and the third one is the one I can recall almost in its entirety, even my thoughts and feelings.. friends loved, and enemies hated. Though it's all jumbled together with the 'me now' and the 'me now' is a completely different person, with a completely different mind.
I'd like to further add to the points made by the others, that the more you recall the past, the more you become unable to live in the present.
The past pollutes you as much as it heals, especially if it's a life you so desperately want to go back to, that you'd die tonight, if you though there was a chance of returning to it.
And another thing that inhibits memory, is the brand new mind you have today, and the society you've grown up in. Different minds can only hold so much of former ones, our brains now don't necessarily work the same way as our brains before. A more developed frontal lobe this time around, or a more creative or restrictive personality trait this time around, different priorities and opinions on things.
Coupled with the society you're raised into, were you raised in Ghana, Africa, where food would be short and lifespans more unpredictable, you would have less time to devote to remembering past lives, because of the constant fight to stay alive in the current one.
However, born in the UK, with a roof over my head, food in my belly and no predators at my back, like many others I can afford the luxury of letting my mind wander to past lives, and other fanciful things.
It's circumstance, condition, time and awareness which either inhibit or allow the growth of the spiritual self and the memory of self.
I hope this helps and I'm not just babbling.