I agree - truly terrible acting, the least convincing policewoman ever seen on TV! And how come at the end when Nicky was outside her front door it wasn't shattered by the bullet her would-be assassin fired earlier?
In one particular way it certainly has gone downhill and that's the storylines.
Most of the last series and now the beginning of this latest heavily involve the characters themselves, their lives and their histories. It's moved from being a series of stories involving forensic pathology to a series of stories about forensic pathologists. In effect it has become a glorified soap.
Many series suffer from this. Though not directly comparable (and certainly nowhere near so good) "The Bill" is a shining example. It started as a series of stories about police cases but when it was finally put out of its misery it was simply a soap involving characters who all happened to be police officers. It's a great shame that Silent Witness seems to be going this way.