Does not being certain about the desire to marry prevent a prayer for marriage to a specific girl from being answered?

There is no harm in supplicating for marriage to a specific woman, but it is better to ask Allah to decree for you what is good, wherever it may be. This is because Allah alone knows the true nature of things. A person might supplicate for something they perceive as good, while it is actually harmful to them, and Allah diverts it from them through His mercy and wisdom. Allah Almighty says: "And man supplicates for evil as he supplicates for good, and man is ever hasty." (Al-Isra: 11) This means that one might supplicate for what they deem good, but it is in reality evil, due to their hastiness and ignorance of the true nature of affairs. Therefore, entrust your affairs to Allah and ask Him to facilitate what is good for you wherever it may be.