To keep lavender from fading I usually add a base note like patchouli, or cedarwood, or litsea cubeba even! but I usually find that lavender eo has great staying power.
Long ago I read that some castor oil in the base oils will help with scent retention over time. So I always add at least 3% castor oil to my recipes. I have no problem with scents fading.
but if that should happen after many weeks or months: a cotton ball soaked in lavender, and placed inside a lidded plastic box with the soap for a week helps perk it up.