The brandish command does not differentiate between offensive or defensive spells contained in the staff.

You can't buy wands, staves or scrolls from shops that you can't use, i.e., they aren't affected by the +5 level difference like weapons and armor.

If the level of the wand, staff or scroll is greater than your level, you won't be able to invoke it (note that the level of the spell contained in the object is independent of the wand level, and can be higher or lower). Just because you find it in a low level area or kill a mob for it doesn't mean the item will be your level or lower. Use identify, or if you get two, find a shop and sell one then type list to see what level it is. Shoppies mark up price, they don't change the level of objects they sell.

Your troubles are most likely either from trying to use staves that are higher than your level or bad luck with the RNG.

Kastellyn the Devourer of Magic, Lord of Legends