This sounds like it might be a classic symptom of air-lock. The "weep" hole as it is called should always be drilled in your discharge pipe to prevent an air-lock. This is true in sump and sewage applications. Air locking on a sump pump happens when there is air between the pump and the check valve of the discharge line. The trapped air prevents the impeller from drawing in enough fluid to open the check valve to allow discharge. The weight of the fluid on the topside of the check valve keeps it closed.
The problem is often noticed after the initial use when the pit has had time to dry out during a dry spell. When it gets water in the pit again, the air-lock condition shows up. The pump runs due to the float switch being raised, but can't eject the water because of the air-lock condition. I suggest that you drill a 1/8" or 3/16" hole in your PVC discharge pipe, approximately 4" above your pump's discharge port and well within the inside of the basin. Drill the hole at a 45-degree�angle so the hole is pointing toward the bottom of your sump basin. Water will be discharged from the weep hole when your pump operates. This is completely normal.
A typical installation will have the check valve positioned about 2-3 feet above your sump basin cover. If the check valve is screwed directly into the sump pump's discharge port, there is really no place to drill the weep hole, but also less of a chance of air-lock due to the positioning of the check valve. The only problem with installing the check valve directly into the pump's discharge port is that it takes more power and effort for the pump to start due to the added weight of all the water sitting above that check valve, especially if your head is 8 feet or higher. When the check valve is installed above the basin cover, it reduces that starting power and also makes for a much easier method of replacing or performing maintenance on your pump and system. If this was done and you are still having problems, I would suggest calling Zoeller tech support at 800-928-7867 for troubleshooting assistance.