RedirectMatch rules are, by default, case-sensitive:

# 'Old-Page.htm' will result in a 404 error
RedirectMatch 301 ^/old-page.htm$ http://new-site.com/

To make a RedirectMatch rule case-insensitive, add (?i) to the start of pattern:

# Will redirect properly regardless of capitalization:
RedirectMatch 301 ^(?i)/old-page.htm$ http://new-site.com/