I's strongly recommend that you chat with, and listen to, your players baldar.
Their imaginations wil fill in lots of details and ideas that you, as one person, just wouldn't come up with! By co-opting some of those ideas into your evolving adventure, you can produce one that feels a lot 'deeper' than you alone would produce. Plus it also helps with the problem of
your simple plot turning out to be far more complex from the
players' perspective, since it will turn out that someone happens to have guessed aright already!
Also, and apologies if this sounds trite, listen to what they want their characters to do. For example, let's say that in the last adventure, you added as part of the treasure a curious statuette. It's not magical, but your description seems to have piqued the interest of some of the players. Well, run with that idea. Let it be known that someone collects those statuettes, and will pay handsomely for more of them. Right there you have a hook, and can apply some of the other ideas given on those websites suggest. Or if the cleric PC really wants to visit the Motherhouse of his Order, adapt an adventure idea to include travelling towards that place. By working in some of the things that interest them, you make the series of adventures feel less like one adventure after another, and more like an unfolding single tale, with many chapters.
I will also add that, like everything else in life that's worthwhile, creating adventures takes practise. The more you do it, the better you'll get. But accept that you'll make some mistakes, so try and get honest feedback from your players. And don't be afraid, if you do screw up, to cut your losses and move on.
Good luck mate!
