Hi @honjun88.. each time you place a vote, your voting power also decreases which causing the subsequent vote to be lesser and lesser. (The lesser your voting power, the weaker your upvote value).

You can check your voting power at steemd.com/@honjun88.. keep your voting power above 80% or limit to 10 upvotes per day for maximum effectiveness.

Your voting power recharges ony 20% per day, so imagine if you deplete your voting power, it can take longer time to recharge to 100% voting power. Golden rules is 'do not abuse ur upvote, as the more u upvote, the lesser ur voting power and the lesser ur upvote value.