I have an old midi keyboard (M-Audio Keystation Pro 88)  which I have used with success as a midi input device.  Recently however it has been working intermittently - at times Sonar recognizes the input, more often at other times it does not.  I have tried testing it in different ways;  connecting the keyboard directly to USB port on computer, and through two different audio interfaces, M-Audio Fast Track Ultra and Tascam US-1800.  In all cases I see the midi input indicated on the computer and on the audio interface.  I would appreciate any and all input to help resolve my problem.  I am running Windows 10 on an HP computer.

Do you have USB Power Management disabled?

Do you use the M-Audio Driver?

I use an M-Audio Keystation Pro 88 without issue here.

Hi. If Wookiee isn't right about USB Power Management, perhaps try a different cable?
I have an M-Audio Keystation 61es and have no issues with it.
All the best.

I have the same intermittent frustrating problem with my EMU X Board 49 under USB.. Was working fine, then used again after a couple of weeks break. nothing seemed to work.. swapping ports etc...USB power management is disabled.. I ended up uninstalling via device manager and reinstalling and it was ok. I used to use EMU's X board drivers with Win 7 .. but Win 10 installs it as plug and play.. Don't have a sound card with midi ports at the moment so don't know if all is well using midi interface.

One other thing I have found with USB Audio devices is they do not like sharing USB ports.  Those two ports next to each other are a shared port, best to only plug in to one of them.

There are only 2 problems I have experienced with USB devices:
1) if you connect the same device to different ports. Even in case that has happened once by mistake. Sonar does not like that. Use Windows device manager with "show hidden devices" set to uninstall ALL instances of that device.
2) bad USB cable/hub. There is zero diagnostic, but the software/drivers are rarely handle errors correctly. The result is unpredictable. F.e. I have a mouse which disconnects/reconnects all the time. Apart from periodical problems with clicking, I could not notice that till I have tried to use it with my own driver...
 

Because of a limited number of USB ports on my laptop I am using a USB hub.  Is that a problem?  Can I assume that that comes under the heading of sharing a USB port?
 
Referring to azslow3 point #1:  I have tried to connect the midi keyboard through the midi in ports of both my audio interfaces (Fast Track Ultra and Tascam US-1800). Do I need to uninstall all instances of these devices?  I do not recognize any device name that corresponds to my connecting the Keystation Pro directly to my laptop.  Any downside to unintentionally uninstalling the wrong device?
 
I have drivers installed:
M-Audio USB MIDI Series 5_0_1 for Keystation Pro 88
FastTrackUltra_6_1_10_Win_83292 for Fast Track Ultra
TASCAM_US1800_2.07rc1 for TASCAM US-1800

Using a hub is not recommended.

If you use the MIDI ports on the Key Station Pro you will not see it named, only the MIDI ports of the MIDI interfaces you have.  In this case you need to remember what MIDI port you have it plugged in to and then select that in CbB or alternately rename the port on your device it is connected to using a friendly name in Preferences > MIDI > Devices. 

You will also have to use an external PSU do not under any circumstances have the USB connected and the external PSU it will destroy your Keystation Pro.

I implemented all the suggestions from your replies.  I unplugged everything from the USB ports.   Connected Keystation Pro using PSU.  Connected midi out on keyboard to midi in on Fast Track Ultra.  Connected Fast Track Ultra to usb on laptop.  I rebooted my computer and power cycled everything. Brought up Sonar and created a new project using ctrl n.  Added Cakewalk SI-Electric Piano as a simple instrument track.  Checked that there was input from the synth property page.  Played the keystation pro and saw that there was input registering on the meters.  Saved the new project and closed sonar.  reopened sonar and opened the project I had saved.  Verified there was input to Si-Electric Piano using synth property page.  Played the keystation pro keyboard.  No input.  I am about at my wits end with this but dont know what to replace; midi keyboard, fast track ultra, or trash Sonar and go to Pro Tools...
 

It almost reads like Input Echo is not enabled