may need to re-restore & re-jailbreak
had the same problem with different PCs - one able to see iphone & other being unable to see phone. Solved by restoring with custom IPSW, rerestoring my phone on PC that could see phone (thus 2 restores) and THEN jailbreaking. I didn't choose to set up as a new phone.
The first time I'd just restored with the custom IPSW and quickly ran thru the phone setup. Upon jailbreaking I got a few errors & lost contacts, etc. It was at this point my other PC was unable to see the phone- however the other one was able to still detect the phone.
Anyway, decided to restore again with the same custom IPSW, but chose to also restore the phone with a backup via itunes rather than set up as a new phone. All went fine (contacts, photos, etc) & I rejailbroke the phone with 14b2. It took a loooooonnnng time to set up but eventually got Cydia, etc installed & working, no errors. Today, I tried the other PC with iTunes which previously couldn't detect the phone- detected the phone with no errors & synced fine. My guess is this is a protection so that if someone doesn't get all your info if the phone is stolen. Also, I noted that getting the phone properly into DFU mode is a key.