Do not confuse slow with stuck
Moulting can take hours, particularly in large spiders. A pause does not necessarily mean the moult has failed. Intervening too early can tear soft tissue and make the situation worse.
Signs of a real problem
A spider that remains trapped in the old exoskeleton for an abnormally long time, cannot free several limbs, is leaking haemolymph or is visibly collapsing may be in trouble.
Do not pull the old skin
Never simply peel the exuvia away. Soft joints and membranes can tear. Improvised force is one of the easiest ways to turn a difficult moult into a fatal injury.
Stabilise conditions
Keep the enclosure stable, ensure water is available nearby and avoid handling. Do not soak or spray the spider directly as a routine response.
Get specialist help
A severe failed moult is a reasonable reason to contact an exotics or invertebrate-experienced vet. Professional help is especially important when the mouthparts or abdomen are involved.
After a partial moult
A spider may survive missing or damaged legs and can regenerate limbs over future moults. Immediate priorities are preventing fluid loss, reducing stress and making sure the animal can access water.