The number of players you can scout per week depends on your scouting agency. In my first year I could scout 19 per week, but in the offseason I had the option to pick from 4 scouting agencies that allowed me anywhere from 18-24 per week. Also, these scouting agencies varied in "experience", which I'm guessing means they differ in the accuracy of their scouting reports.
Anyway, in your case, you can scout 18 players every week. If you decide you want to switch one player and scout another - say you got a report saying WR X has 55 catching, so you want to scout a different WR next week - you can do that. You can scout each player a maximum of 6 weeks before they stop giving you info on that player (or at least that's the number with my scouting agency, not sure if it differs with others).
I agree that scouting could be easy if you master it. I think the problem is that during scouting, players are sorted by their potential rating. This means, you already know the potential of theses players going in. And then, when you get to the draft screen, some guys that you remembered had the best potential at their position will be projected 4th, 5th, 6th, 7th round picks. Obviously these guys are almost always going to be gems. And you know who the busts are too...they're the ones that had low potential ratings that are projected in the 1st, 2nd, 3rd round.
The way to fix this is to not give away the potential ratings of all players. Make us scout them to get their potential, and sort them by something else...maybe "Mel Kiper's rankings" or just a generic "draft analyst rankings". These guys rarely know who the gems and busts are, and are frequently wrong with their projections. This would make scouting more difficult.
EDIT - I just thought that I should also note that in my first draft, I was able to nab a 84 OVR RT with A POT with pick 28, a 81 OVR WR with A POT with pick 60, and a 77 OVR WR with B POT in the 6th.