The steppers in the K40 can be a number of different units depending on what’s cheap in china at the time your machine was produced.
Acceleration, a good rule of thumb, should be decreased in 500mm/sec^2 increments until you get good performance. But first, try tuning the motor current.
Read the model number of your motor off the side of it and put that into your search engine of choice. With luck, you’ll reveal the manufacturer’s recommended peak current value. Multiply that by 0.707 and use the resultant value for current on that particular axis (the stepper drivers on the Laserboard are tuned in RMS current, not peak current as most motors are specified. If your manufacturer actually does list an RMS current value, use that instead).
For the X axis, start at 0.6A and increase this by 0.2A at a time until the skipping goes away (use 0.9A and 0.1A for Y). Be careful, though, not to overcurrent your motor. It’s OK for them to be a little warm, but they should not be hot and definitely not too hot to touch. If they are, allow them to cool, back the current way off, and try again in half increments until they are just warm to the touch after operation. Other ways to tell a motor is over-current is that it’ll possibly groan and strain, make crunching or grinding noises under fast moves, emit smoke, etc. (if you get to the smoke point, your motor’s probably toast).
If tuning the current has little or no effect, start messing with the acceleration values as above. 500mm/sec^2 increments until it stops skipping, then play around with increasing it smaller numbers until you can just barely get it to skip, then back off by 100. (I’ve found that a regular 5-pointed star outline is really good at getting the machine to skip when the speed is set high enough - 200-250mm/sec is a good starting point).
Realistically, due to differences in machine condition and tolerances, you’ll likely be doing a combination of the two (motor currents plus acceleration). It can be a finnicky process. Make sure the belts are tight (but not too tight), rollers are lubricated, and the gantry is square as part of this process.