The strength indicator does not show LTE signal strength, ever. Even when the icon says 4G. The signal strength shown is always for 1x. So you likely had a very weak LTE signal outside and then you went inside and the signal was too weak to penetrate where you were at. I just got back from LTE Performance testing in Waco and I can tell you that the Sprint LTE penetration characteristics are far better than WiMax.
To find out the LTE signal strength, you must go Settings>About Device>Status. Go down to Mobile Network Type and confirm it is LTE. If so, read the signal strength listed above. If it is a number greater than -115dBm on the EVO LTE (or -90dBm on the GS3, GNex, Viper), then you have a pretty weak LTE signal.
More and more coverage is constantly being added every week. If you are just now on the edge of service, you likely will be fully covered in the future. Everyone with an EVDO signal strength of -98dBm or better will likely get LTE service at build out.
Robert