I have several fountains and low voltage transformers in the garden. I think all the outlets are on the same circuit. Sometimes during watering one of the outlets gets wet and trips all the outlets. I can usually reset the gfci and all is well. This time part came on and part of the outlets are not working. How could that happen?

Most likely some are on the load side of the gfci and some are not, call an electrician, this could be a hazardous situation for you