I'd confirm the problem is due to poor adhesion. Us a razor to cut horizontal and vertical lines in your Porter paint to make a checkerboard pattern. Press 2 inch wide ordinary white/yellowish masking tape over your crosshatch pattern and press it down tightly. Now pull the tape off quickly. The more squares of Porter paint that come up with the tape rather than stay stuck to the column, the lower the adhesion of your Porter paint to whatever is under it.
What I'm describing is the standard way paint adhesion to a substrate is tested by paint companies. If most of the Porter paint came up with the masking tape, take that piece of tape down to wherever you bought the Porter paint from and show them the results of your adhesion test. They may give you a refund on the paint you bought.
If it wuz me, I would use a sharp paint scraper to scrape off your Porter latex paint. Or, I would use xylene to remove it from the oil based paint it's on. (use plenty of drop clothes because cleaning latex paint off of anything with xylene can be messy, and you'll drip latex paint all over the place) Then, roughen the oil based paint on your columns by cleaning them with a green 3M Scotchbrite scouring pad (sold in grocery stores for scouring pots) dipped into a strong solution of TSP. Then, rinse the TSP off with clean rinse water, and allow to dry. Now, repaint those columns with a good quality exterior alkyd (or "oil based") paint tinted to the colour of your preference.
Maybe see my response to "Ehoez" in the thread in this forum entitled
Anyone know any good Ext Paint Designs/Color Chocies (for ranch style homes)?
It may be worthwhile reading.