Just as reference, as Vikki says, a little corrosion can go a long way. You can dump all the hotshots and chargers onto the battery terminals, but if there is reisitance at the terminal-to-battery connections, or chassis ground, it doesn't matter how many amps you throw at it.

Also as reference, a starter is designed to operate for about 2 seconds, and 3 to 5 seconds of operation is pushing it to its limit. Operating it for 7 or more seconds will make toast out of a starter fairly quickly. You might not notice the damage on the short timeline, but such operating conditions is the reason starters crap out in 5 to 10 years. If you operate the starter as it was designed to operate, it will last many decades--my starter has been bakeing by the header for 20 years, many of those years as a daily driver, and it still cranks the same as when it was new.

If the car doesn't fire in 3 to 5 seconds, something is wrong with the car, and grinding on the starter isn't the solution.