Gamera

You will find that the most efficient way to improve items is to farm power units. In my factories I only build "cheap but slow" (while I'm at work and overnight) or "chance for rare items", then feed every common and rare item I get to boost and transform my power units. It costs less gold to boost power units with items than to boost items with other items, and you even get a 10% bonus when boosting power units with other power units (I do this to boost epics to max using rares). Once you start doing this you will get power units all the time, I get them faster than I can spend them. Just keep playing campaigns and you will get item boxes, too. 

OK, great suggestion. I'll have to search how to clear cache on a Samsung Galaxy S8. 

You will find that the most efficient way to improve items is to farm power units. In my factories I only build "cheap but slow" (while I'm at work and overnight) or "chance for rare items", then feed every common and rare item I get to boost and transform my power units. It costs less gold to boost power units with items than to boost items with other items, and you even get a 10% bonus when boosting power units with other power units (I do this to boost epics to max using rares). Once you start doing this you will get power units all the time, I get them faster than I can spend them. Just keep playing campaigns and you will get item boxes, too.