• Deck contains no strategy of any sort
• Deck contains few/weak monsters
• Deck contains too many strong monsters
• Deck contains few magic/trap cards
• Deck contains too many magic/trap cards

Solutions:

• Include some kind of strategy to insure your victory
• Include a sufficient amount of monsters in your deck
• Include few five or higher level monsters in your deck
• Include multiple or different magic/trap cards to enhance your deck
• Half of your deck should be monsters and the other half should be magic/trap cards

I hope this have helped you enhance your deck. As long as you follow all the requirements in the solution section, your deck should be fine. Now, show your opponent the true potential of your deck!