Explain how a neurotoxin present in the venom can paralyze one type of prey but not another

This is because some organismsbhave adapted to become immune to these poisons. 

Most neurotoxins are transferred through the blood stream, and use the blood cells to transfer to the nerves. Pufferfish poison, for example, follows sodium channels in the blood. It can paralyze the lungs because it follows Na- channels, but not the Na+ channels that lead to the heart. This is because humans have indirectly evolved, or evolved for one reason that just so happened to help in another way, to become partially immune.

Another example, bee venom won't kill you if you aren't allergic, however, it can kill some other organisms

Following on from what lgodbout23otm0my said, it could be to do on a microscopic level as well and how the tertiary structure of the protiens in the neurotoxins are not specific and complementary to the enzymes in the host and thus do not break down to cause damage to the host tissues from the products of the hydrolysis