Quite a number of times I have heard comments similar to you need to just keep writing songs and then at some point you will get that one song that becomes a resounding success. One Christian writer where most of you would know his songs has said you need to write maybe 30-40 songs before you get one you record. Another writer who almost all of you know has said regarding a recent album, he wrote 150 songs for it and recorded I think ten songs. For the vast majority of Christian worship song writers, there are those elusive qualities of songs that make them popular. So far I have read or listened to over sixty Christian writers give their ideas on how to make excellent melodies and it has been very seldom they have mentioned even one of the qualities mentioned below. One secular book I read has said we have no rules and formulas to derive great melodies analytically. I disagree and no one has successfully proven me wrong on this because it is so easy to verify the information with a bit of work.
1) Worship songs that have only four notes in the vocals are usually the ones that have limited appeal and require a prominent organization, along with a veteran artist doing a hundred concerts a year to make any significant headway. There is a way of using four notes in the vocals with moderate success but I don't recommend doing an entire album with that style. You can find many of those songs in the discount bin at your Christian book store.
2) A song that gets sung in a hundred or a thousand churches on a given Sunday across North America usually has six or more notes of a major scale in the vocals. Some writers work you could examine to verify this are Matt Redman, Keith Green, Chris Tomlin, and many others.
3) A mistake that has been made by those who have six or more tones of a major scale in their vocals is the writer sings four notes of a scale for half of the song, changes the key and then continues to sing four notes of a scale the rest of the song. When the writer has their six or more notes, they need to sing three consecutive notes of that scale portion (such as ABC, or EDC) many times throughout the song. You will find the only moderately successful four notes in the vocals songs have the three consecutive notes of a scale throughout the song. Some of the most successful Christian worship songs have 8,9,10, or even eleven notes from major scales along with the three consecutive notes.
4)An exception to the six notes and three consecutive notes is the secular song One Tin Soldier which does not use a lot of the three consecutive notes but uses the 1,3,5 pattern of a scale to give it that range and appeal. I believe a lot of it uses mostly five notes of a scale.
5) When you sing a phrase of vocals or a verse, an extremely common mistake is to sing for 4 seconds, pause several seconds for instrumental, sing for four seconds etc. The writer needs to sing for six to eight seconds, pause for one second, then sing for 6-8 seconds again but there are lots of exceptions to this.
6) Good worship songs often have a heart felt plea like, "Hear Me Lord," "You are my rock," etc.
7) Good worship songs usually have lyrics that trigger you to think of a bible verse or some other Christian concept. Good worship songs often quote scripture. Heb 4:12 says the Word of God is sharper than any two edged sword which means those words are stronger than our words.
8) If you sing in a coffee house with long instrumental portions in your songs, you will find these are the parts where the audience starts doing a lot more talking and day dreaming.
9) Hymns have a lot of quarter notes and modern worship songs have a lot of eighth notes but there are many exceptions to both. After you have read the above, consider the thought that Charles Wesley wrote well over a thousand hymns but only about 4-5 are popular. The above principles reveal why. When you listen to the 1980's song As The Deer Pants, you quickly see now why it was extremely successful.
10) One person commented to me there is a concept called musicology which analyzes the melodic structure of songs to identify what makes good music. You may want to consider one of these books but since they are likely written by professional musicians, their method of explaining it is likely to be a bit abstract or complex. Amazon sells those types of books and sorry for being long winded. Now you should be able to write about 15 songs and ten of them will make it to your CD with much greater chances of success.