I'm having trouble with a low dropout regulator.  I am trying to analyze its stability by breaking the feedback loop, injecting an ac signal, and analyzing the transfer function (tf=output/ac input).  But, the loop gain I get is always below 1.  Any suggestions?  Thanks very much.

It's not so easy. Simply breaking (opening) the loop is not correct because the circuit looses its operating point. You must study some methods to simulate the loop gain correctly.
One simple and robust method (not 100% correct): Insert an ac source in series with the opamp output and display gain and phase for
the expression db(V(x)/V(y)).   Nodes X and y, respectively, are the nodes to the right and to the left from the ac source.

getting proper stability in an LDO control loop is a bit messy - the output power pass transistor wiill change about 30dB in gain over the full operating range (triode to saturation) the ESR of the loading capacitor affects pole placement. Its not impossible, but it needs to be carefully examined for stability/gain/phase over all operating situations so you don't build an oscillator.