The secret rule in bike gearing is that you really can't use at least two of the combinations on your bike. This is what Al is talking about with cross-chaining: when the chain is on the small front chainring, you will not be able to use the smallest rear cog; likewise, when on the largest front chainring, you will not be able to use the largest rear cog. So your 12 speed is now a 10 speed.
That said, one thing I've found when adjusting shifting on a bike new to me is that the front derailleur often gets tweaked out of line with the chainline / chainrings (twisted on the seat tube). If this is the case, lining it back up could give you one of these gears.