I was shooting my Bushmaster M4 Type for the first time, it was new outta box. (But cleaned and lubed)

I put 100rds. Win 3131 thru. About the 40th round I fired it did not eject the case. And held it in the extractor straight in line with the chamber not sticking out the ejection port and picked up another round from the magazine jamming it. I cleared it and chambered another round from there on out it ran 100% with the next give or take 60rds.

Anyway what might have caused this? I assume the ejector was stuck depressed?
anything I should check out or look for? I realize the rifle needs broke in.
Is this anything to even be concerned with or something that might need fixed if it does it anymore?

Thanks
possum58 (noob)