For my birthday my grand dad gave me his old 3 pickup electric hofner. I really like it but I notice when hammering on it makes little to no noise. Its definitely a hardware issue and not myself as I hammer on fine with my epiphone dot etc.

So does anyone know what causes this?
Sorry I forgot the model name

My guess is that you have a heavy picking hand, a light fretting hand, and the guitar has uneven frets. So when you pick notes they sound, but when you hammer on you do so lightly and the note dies quickly. Find a good guitar tech (NOT GUITAR CENTER) and get the frets leveled.