If you haven't found the service records of your relatives and you are fairly confident that you've used the correct search terms, then the chances are that the records are among the majority which haven't survived. By the correct search terms, I mean that you have tried both precise details, such as first names and dates of birth as well as more fuzzy searches. For example someone born Henry may well appear in the records as Harry, and dates of birth given to recruiters can differ greatly from the actual date of birth - no independent checks were made.
But even if the main service record has been lost, there are other records which have survived. For example some medical records and some prisoner of war records are available, and if any of your relatives were entitled to a pension as a result of their service, the records of this will still exist. Finally some ancillary sources such as absentee voter records for 1919-20, and the 1921 census can throw up some details about a man's military service. Also if you know any of units that your relatives served in it may be worth contacting the museum connected with each unit as they hold other sorts of records. This last approach works best with infantry regiments and less well with the larger corps such as the Labour Corps or Machine Gun Corps.
If you found Medal Index Cards which you are certain belong to your relatives (bearing in mind what AllanUK said) then having each man's regimental number(s) will aid your search using the other sources I have mentioned, and also may allow you to pin down when they enlisted, as the numbers were issued sequentially. Use Paul Nixon's excellent website to help with this:
https://armyservicenumbers.blogspot.com/p/index.html