Why some of the periods are not displayed in print preview?

Most common cause is, that you have selected, that this (or these) period shall be printed only in one of the bells.
Set it back to be printed in "All bells".

Do not worry, if you have set different times for start and end of these periods in other "bells", they will be printed correctly.

See also:
Máme rôzny čas prestávok v jednotlivých častiach našej školy. Ako to zadať? (Riešenie 2)