How to get powerful in low or middle voice?

When I sing Anna Lee - Dream Theater, I can't to get my power in the low part. My voice just disappeared from the band instrument. When I sing Fight For Freedom - Gugun Blues Shelter, I got the same trouble. Is this caused by male voice that I singing? Or any way to get better without change the key center of the song???? Wishing you could help me about it, guys! 
Thankyou~

Thankyou~
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