Brush Tool

Hi @Ess_ Thank you for reaching out and welcome to our community! Could you please tell us the version of your OS and the Photoshop application you're using ?

Adding to what @Ged_Traynor has provided you can also try:

1) Resetting the Brush tool :

Right-click (or Control-click) the Brush Tool icon in the Options Bar.

From the dropdown menu, select "Reset Tool".

2) Resetting all tools :

You may want to reset all Photoshop tools to their default settings, use the same menu and select "Reset All Tools" instead.

3) Resetting Brush Presets

You might want to reset the brush presets themselves (the actual brushes in the Brush Picker), open the Brush Preset Picker, click the menu icon (gear or three lines), and choose "Reset Brushes". This will restore the default set of brushes, but will not affect the tool settings.

4) Resetting your preferences : 

You can try resetting your preferences but be sure to back them up first. This Helpx article would help: https://adobe.ly/4kfE9kK

Let us know how it goes. Cheers!