dvd vcr combi?

[Giggsie]

I have a dvd vcr combi it has always worked well. But since getting new lcd tv sometimes when I insert a dvd it comes up no signal why would this be? Next time I try even next day it will come up dvd and plays ok. Quite annoying if you fancy watching a dvd then have to see if it's a night it's going to work or not. Anyone help me here?