A reward or voucher that fails on a Lightspeed K-Series POS is almost always a Lightspeed setting. You can fix it with the merchant, without a developer. Go through the steps below in order and stop when it works.
Important: after changing anything in Lightspeed, the POS must retrieve its latest configuration. Until it does, nothing changes on the POS, and the change looks like it didn't work.
Step 1: Check the setup
In Lightspeed
The discount code is lower case with underscores, no spaces. Example:
birthday_voucher.All accounting groups are selected on the discount. This is the most common cause.
Under POS → Configurations, the discount is selected in the configuration the iPad uses.
The discount is available to every POS user who redeems Leat offers. Leat applies it as the logged-in user.
In Leat (Apps → Integrations → Lightspeed K-Series)
Click Authenticate with Lightspeed.
On the Sync tab, click Sync items & discounts.
On the reward or voucher, open Discount logic and select the discount.
Choose Applies to (step 2) and select the products or categories.
After any change in Lightspeed: retrieve the latest configuration on every iPad. Otherwise nothing changes.
Step 2: Check "Applies to" on the offer
The whole bill → Entire order
Products with the same SKU at every location → Item across all locations
Products whose SKU differs per location → Item at a specific location
Every product in an accounting group → Item in a specific category
Then select the products or categories.
Step 3: Apply the discount by hand
Ask the merchant to apply the same Lightspeed discount by hand, on the same iPad, the same way as the offer (whole bill or one product). If their staff aren't allowed to apply discounts by hand, a manager or owner account can do the test.
Fails by hand too: it's a Lightspeed setting. Go to step 4.
Works for the manager, fails for staff through Leat: the staff user isn't allowed this discount. Leat applies the discount as the POS user who is logged in, so that user needs the discount in Lightspeed. Go to step 4, first message.
Works by hand: go to step 5.
Step 4: Fix it by the message on the POS
"Could not toggle discount"
Discount available to the logged-in POS user
All accounting groups selected
Discount selected under POS → Configurations
Only one location fails? Compare it with a location that works
"The discount didn't end up on the order"
All accounting groups selected
Offer linked to a £0 item inside a combo? Link the combo or a priced item
Another discount already on the bill? Only one fits
"This discount isn't available on this till"
Discount active and not hidden
Discount selected under POS → Configurations
In Leat: Sync items & discounts
After the fix: retrieve the latest configuration on the iPad, then try again. Without that step the fix doesn't reach the POS.
Step 5: Still failing issue? Escalate to Support
Escalate when it works by hand but not through Leat, or still fails after step 4. Send:
Account ID
Name of the reward or voucher
Time of the failed attempt
Which iPad and location
A photo or video of the discount failing