The usual advice is:
Make sure you have an even coating of ink on your stamp, and brayering the ink on may well help.If using an inkpad, put the stamp upside down & tap over it with the pad.'Huffing' over the stamp after you have inked it, just before you stamp with it, sometimes helps- maybe the ink is drying a little too quickly.
Make sure you apply even pressure all over the back of the stamp- standing up helps,as you can put more weight on the stamp, as does putting one hand on top of the other- don't rock the stamp, but press well all round the edges.
A stamping pad, or a stack of newspapers as a 'cushion' can sometimes help, although personally I find a good, firm surface works better for me.
Is the 'missing' portion of the image(s) always in the same area? Maybe you are just missing this when you press down- maybe you have a bit of a dip in your worksurface, or the surface is 'giving' a little bit - try stamping on a different part of your worksurface, or another surface.
Thats as many as I can think of, I hope something helps
