No longer playing some of my .ape files (monkeyaudio)

I'm sure I had no problems with .ape files in the past, but now some of them won't play (getting "Problem Can't Open File For" messages on display). dbPoweramp hover display for these shows:

Channels: 2 (stereo)
Sample Rate: 44.1 KHz
Sample Size: 16 bit
Bit Rate: 1,411 kbps (CD)
Audio Quality: Perfect (Lossless)
Encoder: Monkey's Audio v3.99
Encoder Settings: Normal Compression

However other .ape files with the same encoding play fine. I have one album in which the first 4 ape files don't play but the last two do!

What could be the problem?