Pam Powell replies: "From what you say, I suspect, as you do, that the wood may be too hard for manual nailing. When selecting picture mouldings, it is worth bearing in mind that Ramin Oak and Ash are very hard woods, whilst Obeche and Pine are soft and therefore easier to cut and nail.
"The pins should not be too long as this can result in bending. The purpose of nailing is to secure the joint while the glue is drying and help to provide a more secure joint over a period of time. The glue you are using should present no problems, but always nail while the glue is wet and the joint is still in the clamp. Don't nail after the glue has set as this can break the bond.
A good clamp shouldn't allow the joint to push open and perhaps a review of this piece of your equipment may be helpful.