If you have booked more than one ticket our system will always try and group the seats together. Sometimes the sequence of seat numbers suggests that the seats aren't together when in fact they are. e.g. 2 seats around a table.
If your seats are not grouped together then either:
On many long distance routes there are usually non-reservable carriages so you may be able to find seats together on the day of travel.
For more information on seat reservations see can I reserve a seat?