Is there a rule for the pronunciation of words with the suffix -ative?

Solution 1:

You're close. It depends on the stress of the syllable before the -ative. If there is primary or secondary stress, then you have an unstressed a in the -ative. Otherwise, the a would be stressed and get its full /ei/ sound. Note that some words have different pronunciation patterns in different dialects.

• NA-tive (0 before)
• cre-A-tive (weak before)
• REL-a-tive (strong before)
• con-SERV-a-tive (weak-strong)
• LEG-i-SLA-tive (strong-weak)
• RE-pre-SEN-ta-tive (strong-weak-strong)
• ad-MIN-i-STRA-tive (weak-strong-weak) but also ad-MIN-i-stra-tive

Here is a link to all the -ative words in the COCA.