Ideally, you will always do all you can to make sure you follow the arrangements that you have in place. Your children will rely on you doing this, it can be really upsetting for them when arrangements are not consistent or if they get to a centre and then contact doesn’t happen.
If your attendance or time keeping for contact is inconsistent this can also be very distressing for the other parent and it is likely that this will not help you to develop a relationship with them that is in the best interests of your children.
However, there will always be those occasions where this is just not possible. The best thing to do it to contact the centre at the nearest opportunity so that they can make the necessary arrangements to re-schedule this in a way that is best for your child. If you do not give them enough notice, it might be their policy to still charge you for the session you cannot attend. It is always worth making sure you are aware of the rules around missed sessions and that you do not cancel if there is any way of avoiding this.