There are a lot of answers here; but I have one more. Heisenberg's Uncertainty Principle gets in the way after a certain point.
The uncertainty principle is that ΔρΔx ≥ ħ/2π ; that is (in plain English) our certainty in momentum, times our certainty in position, is (by natural law!) limited to a very small, but importantly non-zero, number.
Momentum is mass times velocity, which is distance over, importantly, time; and the more dramatic the velocity or momentum gets, the less certain we can be about what it is. (I'm glossing over a lot of statistical physics there, but assume that uncertainty in x or ρ is always a certain percentage of its value.)
This is, again, not because of design flaws; it's proven natural law, and one of the corner stones of quantum mechanics. That perfect certainty does not exist.
That said, beyond a certain point, the navigational mechanism of the time machine may not be able to calculate, or store, where it's going. Anything before 1941 may physically be out of reach of any time machine built in the modern day.
Of course, there are potentially ways around this--like going back in time, building another time machine in 1941, and repeating the process--but that does sound like a lot of work and massive investment in 1941-money.