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.