Expanding canvas size

[hea7]

When I try to expand the canvas size on all 4 sides, PS expands the canvas on either 2 or 1 side. I have done this many times without issue. I have relative box checked and all the arrows are on. 
I am on an iMac with Catalina 15.3. 
I have reset preferenes and that did not work either. 

[rdonson]

Which version of PS are you using???

[Garnick]

When I try to expand the canvas size on all 4 sides, PS expands the canvas on either 2 or 1 side. I have done this many times without issue. I have relative box checked and all the arrows are on.
I am on an iMac with Catalina 15.3.
I have reset preferenes and that did not work either.

Since I'm running PS-CS6 on High Sierra my input here is probably not relative to your particular situation.  However, I just checked on my system and it works fine, whether the Relative box is checked or unchecked.  But as Ron has mentioned, it could very well have something to do with your Photoshop Version.       

[Peano]

You could try the crop tool. Hold Shift-Opt and pull one corner out. All four sides should expand equally

[Garnick]

You could try the crop tool. Hold Shift-Opt and pull one corner out. All four sides should expand equally

For the Crop Tool you are correct.  However, I do believe the OP is referring to the canvas size, not the image size due to cropping. Two totally different subjects.  The canvas is what could be described as the window in which the image resides.  When increasing the canvas size you are actually adding editable area(s) to the image, either on one, two or all four sides, and it has nothing to do with the Crop Tool.   

[elliot_n]

For the Crop Tool you are correct.  However, I do believe the OP is referring to the canvas size, not the image size due to cropping. Two totally different subjects.  The canvas is what could be described as the window in which the image resides.  When increasing the canvas size you are actually adding editable area(s) to the image, either on one, two or all four sides, and it has nothing to do with the Crop Tool.   

Crop Tool does the same thing.

[Garnick]

Crop Tool does the same thing.

At no point did I write that the crop tool cannot eventually do the same thing.  What I did write is that the Crop Tool has nothing to do with the Canvas Size Tool.  Obviously the Crop Tool can indeed add canvas, but in my opinion it cannot do so as efficiently as the Canvas Size Tool. And one rather obvious point not mentioned in the initial reply concerning the Crop Tool is this.  For that approach to work one must first expand the size of the image window in order to see how much canvas is being added.  Yes, a small point, but an important one.     

[Peano]

For the Crop Tool you are correct.  However, I do believe the OP is referring to the canvas size, not the image size due to cropping.

Both methods expand the canvas in precisely the same way. The crop tool does not change the image size.