CAN A TRAIN AND TIME BE NOT AVAILABE??

IF ANYone can help with this question...

we have to exchange our vouchers when we land in Japan and i was told to book in all our destinations with the jr rail pass asap... which i have looked up all the trains and times we need .

so can it happen that i wont be given those times that i need on that day ?

if so then what happens ... we are travelling next tuesday and im stressing as i have written to jr rail but have not had a reply ...

i hope someone out there can telll me ... as the travel agent that sold me the voucher hasnt said anything at all about this that i am asking...

thank you toall...