Your dog used to take the stairs two at a time and hop straight into the boot. Now they stop at the bottom step, or stand at the open boot and wait to be lifted.
Rule out pain before anything else
Pain frequently presents as a change in behaviour rather than as an obvious limp, and refusing to jump is a classic version of that. Adolescent dogs are also still growing, which adds its own reasons to check.
Book a veterinary examination first if any of this is true.
It started in the last 2 weeks, or after one obvious slip or fall.
Your dog favours one side, or is slower to get up after resting.
They yelp, flinch or hesitate at the top or bottom of a jump, rather than simply pausing.
They are a large or giant breed between roughly 6 and 18 months old, when growing joints are common.
This page covers handling and confidence around stairs and the boot. Anything about pain, joints or growth is a question for your veterinarian, not a training decision.
Once pain has been ruled out
Some dogs avoid a jump simply because a surface is slippery, a height feels unfamiliar, or one slip made the whole step untrustworthy. That responds to confidence-building rather than a vet visit.
Make the surface trustworthy
Add a non-slip runner or yoga mat to the stairs, and a rubber mat or grippy boot liner in the car. A dog who has slipped once remembers the surface, not the fall.
Try a ramp for the car if your dog is small, older-feeling for their age, or simply will not commit to the height.
Keep the house lit at the top and bottom of the stairs. Some dogs are avoiding a shadow, not the step itself.
Reward each step, rather than luring the whole way
Toss 1 treat onto the first step and let your dog choose to put a paw there. Feed again for that single paw, then stop for the day.
Add 1 step every day or two, only once the previous step is easy and confident. Going slower than feels necessary is what makes this work.
For the boot, feed for 2 feet in before asking for a full jump. Let your dog get back out again as part of the practice.
Do not lift or force
Carrying your dog past the stairs or the boot teaches them that the space is something to be rescued from, not moved through. It also is not always safe for you to lift a growing, adolescent-sized dog.
Safety note
Get help now if your dog cries out, goes stiff, or will not weight-bear on a leg at any point. That is a veterinary emergency question, not a training one.
Today's version, in one line: book the vet check if anything above applies, then a non-slip mat and 1 rewarded paw on the first step.