In basic terms a Silicon Diode 1N4001 etc gives a forward voltage drop of 0.6v.
A Schottky Diode only drops it by 0.2V.
In some situations that extra 0.4v can be helpful.
To prove if it is actually the cause of your problem just connect the Vpp direct to Mclre without the diode and 10k pullup - if this programs ok then diode may be the problem - but notice you do not mention the 100 nf capactitor.
You could also try the 470R resistor instead of the diode, but have not tried that myself.