Monsanto Co. is again having difficulty engaging rival Syngenta AG in possible merger talks following a failed pursuit last summer. A second attempt at negotiating a deal to combine the two crop science giants began last year after John Ramsay, interim CEO for Basel, Switzerland-based Syngenta (NYSE: SYT), said his company was “interested in any value-adding opportunity that exists for us in this industry, which includes Monsanto and many others.” Monsanto, who was rebuffed Syngenta following an…