There are a few areas of consideration when assessing issues with the AR-15 style firearms.
1.) Is the firearm new from a factory? If yes, is it a reputable manufacture, or fly by night?
2.) Is it a new home built? Is this the builders 1st or 50th?
3.) Finally, is this a used rifle that has been fired many times and just started to malfunction?
Ammo and routine maintenance are other items that should be looked at for all 3 considerations. Is it good quality ammo? Hand-loads? Cheap Ammo? When was the last time the rifle was cleaned?
First things first, clear the weapon, make sure it is safe and preform a function check. If the function check fails and it is a new rifle, you might want to send it back. If it is a new home built check that the springs are installed and installed correctly, in the right direction and place. If it is used and just started happening, check for spring wear and replace as needed. The hammer spring is a common culprit of many issues to include intermittent firing, misfires and light fires.
Bad gas can cause a lot of problems like short stroking which can be caused by an improperly staked bolt carrier key coming loose. Another spot to look is if the gas rings are missing or damaged, this is usually on the used rifles. On home built rifles especially, make sure the inlet and the gas port are properly aligned, tightened, and the correct tube is being used. An incorrect or misaligned gas block is a common culprit.
A failure to eject scenario can be cause by extractor and ejector springs. Would not hurt to have a few of those around the shop. Weak springs can cause the spent casing to get caught in the ejection port causing a stove pipe situation. Tell tail signs of ejector/extractor issues are dented cases, round rim markings like scratches or damage to the stamping at the bottom of the round. Inconsistent round ejection is another sign, if you test fire the weapon 3 times and the brass ejects to 3 very different angles, you have an issue. All 3 rounds should end up in a pile close to each other.
Talk to the person bringing you the rifle with the 3 questions above in mind. This will help you narrow down the issue and get the weapon back to your customer quickly. The AR-15 is a reliable weapon and with some initial questioning and test fires as required, you can quickly diagnose the issues and fix it.
YouTube – Various Michael Bell videos