macOS Inconsistent Printing
As far as I can tell there is no macOS driver available for this printer, so it's unclear from your question what driver you're actually using.

However, the first thing to try is to totally reset the printing system on your Mac and then to use the driver for the Samsung ML-2240 printer instead.

Before commencing, download and install the version 2.6 Samsung drivers directly from Apple.

Then follow this process:


Go to Apple > System Preferences
Click on the Printers &amp; Scanners preference pane
Right-click on the list of printers on the left-hand side and select Reset printing system... 
At the next prompt, click on the Reset button
Now go through the process of adding your printer - feel free to choose “Samsung ML-2510 Series” but then use the following steps to add the Samsung ML-2240 printer driver


Click on the “Choose a Driver” dropdown list and choose the Select Software option
Go through the list to select the ML-2240 option

Complete your setup and test to see if this rectifies the problem


Hopefully this will solve the issue for you. However, if it doesn't, you can also repeat the first four steps above and then try setting up the printer using the CUPS option.