LG Volt and Samsung Galaxy S5 Sprint Spark tri-band LTE phones are not connecting to LTE in locations where the Virgin and Boost coverage maps indicate LTE is available. Older non-Spark LTE phones like the Samsung Galaxy S III or Galaxy Victory do work in these areas.
Spark devices not getting LTE in all markets is a temporary issue caused by the way Sprint is rolling out Spark and LTE. Older Sprint single band LTE phones like the S III can connect to CDMA for voice and LTE for data simultaneously. That technology, which is called Simultaneous Voice and LTE (SVLTE), is not supported by Spark phones. Spark uses a different technology called Circuit Switched Fallback (CSFB) to temporarily shut off LTE and enable CDMA when the user makes or receives a call. As soon as the call ends, CSFB switches back to LTE.
Because their phones don't support Simultaneous Voice and LTE, Spark phone users can't use data to look something up on the Web while in the middle of a call. If that wasn't bad enough, CSFB requires network upgrades that haven't been made in all Sprint LTE markets yet. A Spark phone in a non-CSFB market disables LTE and defaults to CDMA/EVDO only mode so that voice calling works. A partial workaround is to change the phone's network mode to LTE-only in settings, which should let the phone connect to LTE data, but will also make the phone unable to make or receive calls.
The good news is that Sprint is working to quickly enable CSFB across its entire network. Click here for a spreadsheet from S4Gru.com showing which markets Sprint identified as having problems as of 12/4/2014. Click here for an S4Gru.com forum thread where users report CSFB related LTE issues, which still seem quite widespread. For a detailed explanation of the technological issues involved see this S4Gru.com article and this Geek.com article.