Welcome to HBT.
The closer you get to a full-boil batch (6-7 gallons), the better you can control all the things that influence brewing. However, there are folks that love no-boil kits, where essentially you're adding pre-hopped syrup to hot water, pitching yeast and calling it beer.
So, you make a gallon or two of very strong wort, then add 3 gallons of top-off water ? - many folks do that with reasonable results.
post your recipe and your process and we'll help determine what might have gone wrong.