short answer
Check that you are logged into the right account, that you are on the LEGO.com site for your account's region, that you have enough points, that there is only one reward in the order, and that the reward code is still valid (codes last 60 days). If the reward doesn't apply, it may be sold out.
This is the checklist that LEGO's help page gives for redemption problems, with a few notes of my own. Go through it in order, because the first four fix most cases.
1. Check you're logged into the right account
If you have more than one LEGO account, make sure you're signed in to the one that holds your Insiders points. If you can't remember which one it is, use the sign-in recovery option on LEGO.com.
The region of the site you shop on has to match the region where your account is registered. An account registered in the UK can't redeem on the US site.
3. Make sure you have enough points
Each reward and each discount has its own cost, and discounts have a minimum, for example 650 points for $5 off in the US. See the points value guide for the minimum in your currency. Points under the minimum can't be used yet.
4. How many rewards can you use at once? One per order
Only one reward can go in an order, so adding a second one is a common reason a code fails. If you want to redeem several, place separate orders.
5. Check the code hasn't expired
Reward codes are valid for 60 days. An expired code can't be used. LEGO's help page doesn't say whether the points come back, so contact customer service and ask.
6. Check the reward isn't sold out
Insiders rewards are available while supplies last. If a reward doesn't apply to your order, it may be sold out, and you'll have to wait for LEGO to offer it again.
If none of the above applies, contact LEGO through the LEGO help center. Have your account email ready and describe exactly what happens, including any message you see.
Two other things that stop points working
- Where you shop: LEGO Certified Stores (marked with a yellow flag) and other retailers don't earn or redeem points.
- Expired points: if you went 18 months without earning or redeeming, your points may have expired. See How long are LEGO points valid?
What are your points worth?
Use the points calculator to see the value of your balance, or read LEGO Insiders program explained.