What is the difficulty now Japanese manufacturers are facing with?

Japan posted an unexpected trade surplus for the first time in three months, adding to signs that manufacturers are recovering from the huge disruption caused by the March 11 earthquake and tsunami.
    Companies so far appear to be coping with power restrictions during peak times by shifting some production to weekends. However, it is unlikely to be plain sailing for Japanese exporters. (21) There remain concerns about power shortages ahead, particularly during the peak summer month of August.