Hi Jill, Portulaccas love hot dry conditions; they even like sandy or poor soil. So if you are treating them like most other plants this indeed could be your problem. Hopefully you have caught the situation in time, they are lovely plants. If they are in a bed with other plants that need good soil and moisture, move them so they can be in full sunshine and dry soil. Clay pots are great for these plants because they dry out so quickly.
If you correct the problem and get them to come back into full bloom here is an extra tip: One year we had them in pots on our deck and they would close their blooms in the afternoon. We were having a party in the afternoon and wanted them to be blooming. So we took them to a shady spot, covered them with black plastic bags, loosely, and they were fooled!! We had lovely pots of blooms for the party.
Keep in touch and let us know how you do, Bye, Diane