I reckon it is the random number generator used to create convoys, any convoy below a certain size just does not appear so if you get unlucky you can miss several convoys. This can be a game killer for the russians and it is a pity that LUCK is playing such an important part in allied pp generation.
A possible improvement would be to have the random number generator minimum number increased from zero to the current minimum convoy size. At least then you would see 3 convoys generated every convoy cycle.
The laws of probability dictate that just because you had a period of low number generation this has no effect on subsequent chances. Any points lost in non generated convoys are lost permanently just as if they had been sunk!