Adjusting the length of your shoulder strap, should be once and done.
But maybe you've experienced this or know someone who has.
(About strap types, I'm "not" referring to genuine leather straps with punched holes)
I'm referring to straps that are made from fabric, faux leather, pre-manufactured webbing, etc. This type of strap includes a metal buckle that you pull the strap material "through", creating a loop. This strap type is very flexible and makes it easy to fine tune strap length. It does not adjust in one inch increments like a strap with punched holes. See image below.
Okay, you're ready for the events of your day.
You shorten the length of your crossbody strap to suit your needs.
Next, you add "stuff" to your bag (increasing its weight).
But, after walking five minutes, there's a problem.
Your bag is hanging too low, your strap is fully extended.
What happened? Why didn't the strap buckle hold its setting?
Most likely, it's because the fit is too loose between your buckle and strap fabric. Refer to the image below, specifically the strap and buckle on the left, note the gaps between the buckle and strap fabric. If the fit is too loose there is not enough contact and resistance in the buckle to "lock" the fabric in place.
Note the difference in "spacing" between the buckle and the fabric for the strap on the right. It is tighter fitting, this strap will hold its setting.
Also note the difference in the thickness of the strap material. The strap on the right is thicker because it is made from two pieces of fabric that are sewn together. The strap fabric on the left is pre-manufactured webbing, it is thinner and explains why there are gaps between the buckle and the fabric.
If you have a strap that is not holding its setting, here are three suggestions.
1) The quickest fix is not the most attractive, but it's still effective. First, adjust your strap length to suit. About 1.5 inches from the buckle, install a safety pin through two pieces of fabric to "lock" them together.
2) A more attractive fix is similar to the safety pin idea. First adjust strap length to suit. About 1.5 inches from the buckle, using matching thread color, sew 3 to 4 stitches through two pieces of fabric to lock them together.
Refer to image below.
3) You could replace the buckle that is currently on your strap. Buckles are inexpensive and come in a variety of sizes, refer to the image below. Source a buckle that is 1/16 inch, or 1/8 inch smaller in length and/or width, this depends on your situation. Keep in mind, it could be changing your buckle size length, or width, or both. But note, it's not a good idea to use a buckle that is more narrow than your strap width, because your strap fabric will bind and not easily pull through the buckle.
If you need help with sewing, here's a suggestion.
Visit your local dry cleaners and ask if they have part time staff who does sewing alterations. Even if they don't have someone to help you, they probably know someone who can.
Hope this helps! 
ps- We help you carry your essentials, our Cell Phone Bags are available in four colors, black, brown, blue and gray, our bag straps are not made with pre-manufactured webbing, they are made using the same fabric as our bags, two pieces of fabric sewn together!