Hello Pam, I was just about to refer you to a Pedigree Resource File when I realised it was yours.
I am replying just because I checked this couple before for Jake, short answer to your question re: Agnes Melville, is I just don't know.
If you look at Scottish Naming Pattern, you could say possibly.
1851 shows three children James, Susan and Agnes
1861 shows four children James, Susan, Elizabeth and Andrew
We know that they had a child David in 1840 also,
So children would be David, James, Susan, Agnes, Elizabeth and Andrew
I know it is not scientific but they seem to have followed SNP in some part, First son David for his father, first daughter Susan for her mother. 3rd daughter for mother Elizabeth. Possibly 2nd daughter Agnes for his mother??
There was no John (her father) or Margaret (Meldrum) that I could find. I assume that David died after 1841 but before 1851 as he doesn't appear again. There would have been time between David's birth 1840 and James' birth 1845 for a John, likewise for a Margaret between Susan and Agnes' birth. If there was a John and a Margaret, they could have died also as their births would have been between census periods.....
The Falkirk Parish Burials pre 1855 does show a Margaret Jack, aged 2 and buried 29th September 1844. There is also a Margaret Jack, no age given who died in 1841. No David and no John of the right age.
On the other hand, I couldn't find anything to show that David Jack and Agnes Melville had a son James. Given the marriage of David Jack and Agnes Melville in 1822 in St. Ninians and the birth of James Jack c. 1820 in Falkirk, it doesn't look likely ...
Incidentally, the 1841 census shows only one David Jack born 1840 in Falkirk. His parents are shown as James Jack and Elizabeth Ried (mistranscribed on Ancestry). I checked the orginal on SP and her name was Elizabeth Reed, there is also a 9 year old James Stratton (I think Stratton) living with them. Not suggesting that Elizabeth Burt was really Reed, only that they may have some relevance to your Jacks.
Funnily enough, the only incidence in my tree where a parents name is completely wrong on the death certificate occured also with a family of Nailers from Camelon ... so it does happen but if I had to put money on it, I would say that I think it is unlikely that his parents were David Jack and Agnes Melville.
Sorry not much help,
Lindsay