There can be mulitple reasons as why the pH meter is not holding its calibration. They include:
Coating on glass electrode: Any coating that forms on the electrode will change the value the meter displays when in the pH buffer. Typically, the change would affect all buffers, so a change that is observed in pH 4 would also be observed in pH 7. This type of change is known as an offset error.
Contaminated buffer:The calibration of a pH meter is as only as good as the buffer. If a pH buffer value changes then the value displayed will change. Typically a contaminated buffer will affect only one calibration point. This will change the slope of the electrode. This should not be a problem as long as the buffers are fresh (less than 6 months old) and changed frequently. For example place buffer in a small baby food jar for daily calibration and change out at least once/week.
After using a cleaning solution it is important to place the probe in storage solution for 2-3 hours before performing a calibration.
Rinse the probe with purified water (distilled, reverse osmosis, deionized) water when moving the probe from one solution to another. For example I have my probe in storage solution then I would rinse and then place in a calibration buffer. If performing a two point calibration then rinse between buffers. After calibration rinse. After taking a reading rinse before placing back in storage solution.
Always store the probe in a solution. It is important to maintain the hydrated layer on the glass and keep the junction hydrated as well. If you do not use storage solution then use pH4. If you do not have pH 4 then use pH 7. Storage solution has microbial inhibitors to keep stuff from growing on the glass. pH 4 is the next recommendation since bacteria do not grow well at lower pH's. For this reason pH 4 is recommended over pH 7. NEVER store in purified water.
pH probes do not last forever. They have a life of 1-2 years. If you have a pH meter with a mV mode the offset/slope can be checked. As long as the offset/slope is acceptable then the probe can be used indefinitely.
Thanks for your response. I've pretty much determined that either the electrode is going bad or I have a bad bottle of calibration fluid. The only item that I wasn't doing correctly per your post was using tap water instead of purified water to rinse with. I've since adjusted that part of my process and still have the issue during second point calibration. It seems to be working ok and it will calibrate so I'm going to continue using it as is. I'll be switching to new bottles of calibration fluid here soon, if it continues I think I can narrow it down to a failing electrode and I'll switch that out.