it will be down to them possibly been jaded or if you've let go a few coaches and not replaced them, then naturally they will start going down, or maybe your training is to intense for the stage of the season hence jaded players and declining attributes.But it will start coming round and it wont be a problem, if you haven't already sign top top coaches then both the jadedness and the decline should stop and they should improve, It happened to me with de gea on fm13 I had him looking like a total beast then he start to decline at 29 then I realised the intensity of his specific training had been on very high since the start.