Raglan seams and chunky yarn

I’m a newcomer to this forum and also a new knitter. Why then, you might ask, would I choose to knit two sweaters (for my son and father-in-law) that have to be ready this week? My question to all you awesome knitters has to do with raglan seams. I’m finally putting together the adult sweater that was knit with very chunky yarn using 10 mm and 8 mm needles. I’ve just tried completing my first raglan seam using mattress stitch and it just looks odd. Any thoughts on what I might be doing wrong or ideas about how to create a neater looking raglan seam? Thanks in advance and also for all the fantastic wisdom I’ve already learned from reading this forum.

The first time I did mattress stitch was on a raglan and I was thrilled how it turned out. I suspect that the chunky yarn may have something to do with your dissatisfaction. Maybe you can try with a thinner yarn the same color?