Not true, a player's value can increase or decrease regardless of contract renewal. The main influence are the player's performances and his original value.
For example if you buy a player for 5M, and he performs at a top level you will easily see him valued at 10M at your second season with that player.
But if you buy a player for 30M, and he rarely plays (if he's a wonderkid) or if he plays but does'nt perform excellently, his value will decrease a bit.
I'm pretty sure this is it so I can't really tell why your striker's value hasn't gone up. How much did you pay for him? For how long have you had him? If you have just finished your season try to wait until the beginning of the second season to see how it goes.