Thanks for writing. Most primroses are cool-season plants and suffer when temperatures get warm, especially at night. The plants are beautiful for spring and usually fade in summer, so it could just be a part of their natural cycle.
But, most primroses do best in moist, well-drained soil that's rich in organic matter. Give them part shade; in most areas, they don't like to get the hot afternoon sun on them.
---Justin, Senior Garden Editor, BHG.com