This isn't the first time that Solarplicity has been mentioned in the news - in February, Ofgem banned the energy supplier from taking on new customers due to its poor switching process and customer service.
The energy regulator also banned the company from increasing vulnerable customers' direct debits, warning that the firm could see its licence revoked if it did not improve.
Ofgem said it had been seeking improvements to Solarplicity's customer service, "which has been poor for a number of months".
It said that between March 2018 and September 2018, "there was an unacceptably high proportion of calls abandoned and unacceptably long call waiting times".
"Whilst call handling has improved, Ofgem has not seen the required improvements elsewhere."
The regulator said that the ban would remain in place for three months unless it saw significant improvements in the company's customer service and switching process.