Doing worse on more recent PTs? Help!

At the beginning of my studying I was testing using PTs from mostly the 50s and 60s. As I'm nearing the October test date and taking more recent exams, my score had been going down - from around 172 consistently to around 168 off-and-on in PTs from the mid-80s. Anybody have any advice on why this might be or how to fix this? It doesn't seem to be any one section that is falling in particular