When plants are planted in a new site, there is commonly an initial period where they grow very little. Trees, for example, may stand in a new site for several years without any obvious new growth. (The transplant-shock period is longer for larger-caliper trees, and shorter for smaller, younger trees.) They take time to establish their root systems in their new site. It sounds like your full-sun site is ideal, and you amended the soil at planting time to make the site's soil more hospitable. Unless you need to move it, I think transplanting it would only be a setback for the Crapemyrtle's growth.