Just look at this expanded W-L record:
overall -- 44-53, day games -- 23-27, night games -- 31-26, one run games -- 12-21.
The Pirates, by comparison, are 14-16 in one run games. The Cubs probably have the worst record in one run games; this points to lack of execution in all phases of the game. Particularly, getting sacrifice hits and productive outs are a big factor, the Cubs just do not sacrifice well with either fly balls, or bunts. Middle relief is also a big issue. Marmol hasn't been great, but he hasn't been the worst either. I remember when the Cubs won the NL Central, their execution was very good and scored a lot of runs with sacrifices and productive outs; things they're not doing this year.