You are getting yourself confused (don't worry, happens to all of us now and then!)
The pressure at the pump is determined by two things:
- the flow rate (pressure is directly proportional to flow)
- the flow resistance of the system.
The flow resistance is (usually) dominated by the column, and will depend on:
- the dimensions of the column (directly proportional to length, and inversely proportional to the square of the column internal diameter)
- the particle size of the packing material (inversely proportional to the square of the particle size)
- the viscosity of the liquid (directly proportional to viscosity). Viscosity depends on liquid composition and temperature.
Aside from that, things like plugged filters, or kinked or very narrow tubing can contribute to flow resistance.
"Isocratic" is just a fancy word that says mobile phase composition and flow stay constant during any given run. So pressure should stay constant during an isocratic run.