Contradiction with James 4:3
This verse contradicts 1 John 5:15 as it states that prayers can go unanswered if they are asked with wrong motives, whereas 1 John 5:15 implies prayers asked according to God's will are heard.
James 4:3: Ye ask, and receive not, because ye ask amiss, that ye may consume [it] upon your lusts. [lusts: or, pleasures]
Contradiction with Matthew 7:7
This verse suggests asking will always result in receiving, while 1 John 5:15 indicates the necessity of conforming to God's will to be heard.
Matthew 7:7: Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you:
Contradiction with Psalm 66:18
This verse says that if one harbors sin in their heart, the Lord will not listen, which contrasts with 1 John 5:15 that asserts God hears us.
Psalm 66:18: If I regard iniquity in my heart, the Lord will not hear [me]: