I was wondering if anyone can help me find out what happened to Martha .Any assistance would be much appreciated
Firstly, how sure are you that this Martha Kerr married Alexander More? You appear to have attached the birth of a daughter, Agnes More, to Martha's record - but have not added him as a spouse. Also, although quite possible, I note she would be around 46 at the time of Agnes' 1881 birth and that this event took place around 40 miles from Martha's own place of baptism.
The best website for finding details of Scottish ancestors / relatives is found at https://www.scotlandspeople.gov.uk/?gclid=EAIaIQobChMI8baGrvf95AIViLbtCh2ioAZnEAAYASAAEgKFLPD_BwE. It is free to register and make basic searches, so I would advise you do this. However, payment is required if you need copies of certificates and other documents.
Martha's death / burial and census records could appear in the records under the name of her husband, even though it was common for Scottish females to retain their maiden name.
The best approach is to see if you can find other sources to complete documentation of the family. I quickly located two - the 1841 census appears to be a suggestion on Martha's detail page and you should attach that to all family members to document their location in 1841.
It appears that the following link is the 1871 census for the mother and some of the children --- https://www.familysearch.org/ark:/61903/1:1:VBL5-R8L
Again you would need to carefully evaluate to determine if this is the same family. If so it appears the husband has passed away before the 1871 date so I would look for his death information and attach any sources found. If you could locate the 1851 census that might tell you if Martha was with the family or not.
There is a record for a Martha Kerr that emigrated at age 18 to the US. Have no idea if this might be Martha https://www.familysearch.org/ark:/61903/1:1:275T-BHD
Good luck in your quest. I haven't answered your question but maybe some ideas that will help you find the answers