My DM lives with us, and has since my DC were 10/11 - they gave her something to live for on a day to day basis, when we lost Dad and was getting herself back together.
She sees my DB's DC maybe 3/4/5 times and year, and loves them just as much. But on a practical note, they simply don't feel as comfortable with her, as they don't know her very well, and to a certain extent, visa versa. They have their own lives, school, hobbies, parties, and understandably dn't want to spend regular weekends travelling a couple of hours to see their Grandma, but it definitely has an impact on the closeness of their relationship.
If you DPs are short-tempered and irritable with your DC, I would try and talk to them about it calmly, and suggest that as time together is limited, they try and be more positive.
One other thing, if you're away, your DP presumably stay with you 24/7 when they're visiting, and we can all get mardy when with others all day and all night. Maybe they see your DN more regularly, but not for long spells of time, so don't get riled with her?