the next...1:
Some additional random thoughts.
Are you watching the pitcher warm up every inning [especially the first] Often times he will throw all his pitches in the first warmup. Watch carefully to see if he tips off the curve ball with a slightly different arm angle, a certain turn of his lead foot, etc.
Can you see the spin/thread rotation on the curve ball as it come out of or somewhere in flight after it leaves his hand? Can you see a split finger pitch [more white than normal] immediately before it leaves his hand. If you are not looking at the hand the micro-second the ball is delivered, that might be the problem. If you are looking and cannot see the curve ball spin at least somewhere in flight then you may have a vision issue as Midlo Dad has suggested. If you see them coming but cannot adjust either to the timing change or the movement of the ball or both, there could be a variety of possible tips to help.
I will mention one I have used for timing issues [but I am not a "hitting coach" other than with my son and a few players I have coached during the years] and, hopefully, others will add more or chirp in that I am wrong. If you have a buddy that can throw a curve ball, have him throw you 10 fast balls in a batting cage followed by 10 curve balls. Swing at every pitch, regardless of location. Don't swing at the off speed pitch until you SEE it is a curve ball. Have him do this three or four times. Ask yourself, how long is the ball in the air [or where is the ball on its trajectory to me] when I can recognize it is off speed or a curve ball. Once you KNOW where that is, then work on adjusting your stance or trigger, hip ****, whatever you use to start your swing so that you provide yourself the opportunity to make the adjustment when you recognize it but still be able to hit the fast ball if you don't recognize the off speed pitch. Widen your stance, shorten your swing, take some movement out of your trigger mechanism, whatever you feel you can do without destroying your fastball swing. Once you get that worked out, practice, practice, practice until you acquire the skill to make the adjustment to the off speed pitch without thinking about it.
If you have not done so already, you might want to read what PGStaff said on this site in the Hitting thread "Question about the mental approach to hitting?", keeping in mind you will only really use this newly dveloped skill when you are in a count where you are NOT looking only for a fastball in your zone.
Good luck next.