Step 1, if you have not already done so, is to go to Logic's preferences and enable the advanced features.
On the channel strip with the guitar sound, click on the EXS 24 to open its window.
Near the top right corner, find the "edit" button and click it.
When the edit window opens, look near the bottom and see the small rectangles that represent the individual key zones. Play the note that has the sample you like, and see which rectangle lights up blue. Select the rectangles that DO NOT light up, and hit delete.
Click/hold the edge of the remaining rectangle, and drag the ends out to expand the range of that key zone. You should now be able to hear the sample play on all the keys covered by the rectangle that you resized.