Fast fashion stores like Forever 21 or H&M may have difficulty with "bringing in younger shoppers, who tend to have less to spend." While these stores generally attract younger shoppers due to their trendy offerings, younger consumers may have tighter budgets, making it challenging for these retailers to maintain high sales volumes if their prices do not align with the financial capabilities of their target demographic.
The other options—attracting customers who want moderate quality clothing, using sales and promotions to entice customers, and having a lot of turnover in inventory to promote fashion trends—are generally strengths of fast fashion brands rather than difficulties.