I notice that when I try to come down on a string with my finger more or less perpendicular to the fretboard, the curve of my fingertip frequently causes my finger to slide to one side or the other of the string. However, if I press down on the string with my finger at an angle to the fretboard, a flatter section of my finger comes in contact with the string and my finger stays put. Unfortuneately, that makes it more likely that I am going to be brushing against adjacent strings. I believe the problem is worse now that I have callused fingertips because they are less grippy than a fleshy fingertip. The problem also appears to be worse on the thicker strings. I believe this is easier for my fingertip to be indented and surround a narrow string and not, therefore, slip to either side of the string. I'm wondering if anyone else recalls having this problem and, if so, what you did. I am wondering for example if coated strings are more slippery and therefore might be accentuating the problem. I am also wondering about some sort of finger tack to make my fingertips more grippy. Bank tellers use something like this to count money I believe.