How soft is it meant to be? Pre-mix and post-mix? Every time I buy some it is very hard. But in that recent video the Perry's did for "How it's Made" the putty Michael is using seens very soft.
The older it is (and the colder it is) the harder it will be. Working it will warm it up, which deals with the temp issue but as far as I'm aware there's nothing you can do if it has aged (though you can slow down ageing by refrigeration).
The softest pack I ever bought was from GW, used immediately: it was also the most expensive! Mind you, it was so soft (and sticky) that after mixing I had to let it stand for a bit before I could do anything useful with it.
The individual 'tubes' of yellow and blue will last longer than the bi-ribbon as there's no mixing (contamination) due to contact in storage. That said, I find the bi-ribbon to be more convenient.