Ever since I started going to the gym a few months ago I have noticed something. Basically over a few weeks I will build up to a certain point but I don't seem to be able to get any better. Once I reach that point I start going downhill again. For example I will build up to say 25 minutes rowing over a few weeks and will then not be able to go any further and will start dropping to only being able to manage a maximum of 5 to 10 minutes continuous rowing. It has been the same pattern no matter what I am doing.
I don't really know all that much about the "technical" side of exercise or bodybuilding so it may be something really obvious but I don't know where else to look so I would be grateful for anything. Information, links, advice etc.