By Erwin SebaHOUSTON (Reuters) - A tentative national agreement to end a six-week strike at twelve U.S. refineries has struggled to win ratification at eight plants as workers and companies struggle to settle local issues, according to union officials.The deal reached on Thursday by the United Steelworkers (USW) and lead industry negotiator Royal Dutch Shell Plc to end the biggest walkout of its kind in 35 years is showing signs of quick passage at just four plants.The four sites where the deal is finding support and progress is being made on local issues are at facilities owned or co-owned by Shell.Workers at these plants, which include three Motiva Enterprises refineries, have scheduled or are expected to hold votes this week to ratify the agreement. They could be back at work next week.But local labor talks were stalled at other plants as union chapters try to address issues not covered by the national deal that defines rules for pay, healthcare and safety.The delays mean companies will have to continue relying on temporary workers to keep their plants running. The stoppages affected plants with a fifth of U.S. crude processing capacity."(The)fight continues at Tesoro, Lyondell, BP, Marathon and others still face challenges with local issues," the USW said Sunday.Lyondell Basell Industries broke off negotiations on Saturday with the USW chapter from its Houston refinery, the union said.The company halted talks claiming 'the Union needed a cooling off period,'" USW local 13-227 said in a letter to its members.It said the company negotiators provided no further explanation. Lyondell spokesman George Smalley said the company would not comment for now.A source familiar with the talks said the company may be waiting to see if more workers cross the picket line at the plant. About 10 percent of the more than 400 workers represented by USW local 13-227 have returned to work there.On Friday, the USW said a strike at the Marathon Petroleum Corp's Galveston Bay Refinery in Texas City, Texas, would continue at least until the company agrees to accept the national settlement reached last weekA Marathon spokesman said the company would be meeting with the union but would have no further comment.Other strikes are at three Tesoro Corp West Coast refineries, BP Plc's Whiting, Indiana refinery and BP's joint-venture refinery in Toledo, Ohio with Husky Energy.(Editing by Terry Wade and W Simon)Join the conversation about this story Click here to read full news..