you should print your revised tax return, for comparison against your original return,

and also your form 1040-X tax return.

This will tell you what is going on.

I did and I found that they have reduced one the credit from 2500 to 500 but I don't know why?

when you amend, various credits can change. This is not unusual.

You don't say which credit was affected.

Review the rules for qualifying for that credit.

First ignore the "Refund-O-Meter" as that will only confuse you.  

Look at the form 1040X column B ... are there any amounts on any line ?  If so which line ?  Did you remove a dependent under the age of 17? 

Look at the form 1040X …

Column A should have the figures from the original return, Column C the corrected figures and Column B the differences between the other 2 which needs an explanation on page 2 of the form 1040X.

If you completed it correctly you will see an entry on either line 16 OR 18:

On line 16 should be the amount you paid with your original return. 

On line 18 should be the amount of your original refund you received.

Then you will see your extra refund on line 22 OR  the new balance due you need to pay on line 20.