For HB to be paid for any period the claimant must have a 'live' claim.
This can be a problem when claimants are away from home, especially if they have notified the DWP / Job Centre of a different address. This is because the DWP / Job Centre will send a cancellation notice to the HB Office who will then suspend the HB claim and write to the claimant at their last known address.
If the claimant fails to respond within a calendar month, the claim for HB is terminated.
If this happened the claimant must submit a new claim and request a backdate. Be aware though that for working age claimants the maximum backdate from date of request is only one month (since April 1st 2016).
If the 'gap' is greater than the period over which the HB Office can backdate the claim, or you do not think that a working age claimant has 'good cause' for a backdate then consider the alternatives.