The current impasse seems to arise because the various train and freight operating companies have competing rights which are unable to be timetabled given the current infrastructure on the East Coast Main Line. So I thought it might be useful to go back in history to look at the decision to grant those rights.
In 2016 the Office of Rail and Road approved the current track access rights for the East Coast after detailed analysis of the various competing claims.
In a letter of 24 March 2016 ORR set out their understanding of the various capacity constraints existing at that time and the services that could be possibly delivered following an all parties meeting earlier that month.
Looking at this it would appear that the expectations for infrastructure improvements have not been achieved (such as at Woodwalton).
In ORR's decisions letter of 12 May 2016 there was an additional comment about the uncertainty of DfT financing.
https://www.orr.gov.uk/sites/default/files/om/2016-05-12-ecml-decision-letter.pdf