You need to ensure that your abilities that have a cooldown are further up the casting list then what your spam ability is. If you don't, you will always simply cast the spammable ability as it is never on cd.
The spell rotation of your macro works on a preference system of top to bottom. If your very top ability never goes on Cool down then the macro will never work past the first line.
Your macro should look something like this
#show Void Bolt
cast Neddra's Grasp
cast Radiate Death
cast Void Bolt
Void Bolt is the spammable ability, hence why it is on the bottom line of the macro.
The other 3 abilities all have cooldowns, so as each ability goes on cd, the next ability on the next line is used.
I hope this answered what you were hoping to achieve.