I have this probleme from time to time with my Samsung Galaxy S21+ where I cannot send picture messages using the Messages app to other Android phones on Verizon network but have no problem sending to iPhone on Verizon network. They start to send, get stuck and then are automatically canceled. Have tried restarting phone, clearing app cache, using mobile data only and nothing works. When I view details on the pictures I'm trying to send it shows File Transfer but when sending to iPhones it shows Multimedia Message.