Standard Deviation of Random effect is 0?

#1
have a model with two random effects

lmer(TotalPayoff~Type+Game+PgvnD*Asym+(1|Subject)+(1|Pairing),REML=FALSE,data=table)->m1

each pairing contains 2 subjects and each Subject is in 2 different pairings with different subjects. For some reason the subject random effect has SD =0 I do not understand this? Is it something wrong with how I structured my random effects?

> m1
Linear mixed model fit by maximum likelihood
Formula: TotalPayoff ~ Type + Game + PgvnD * Asym + (1 | Pairing) + (1 | Subject)
Data: table
AIC BIC logLik deviance REMLdev
1014 1038 -497.8 995.6 964.4
Random effects:
Groups Name Variance Std.Dev.
Subject (Intercept) 0.000 0.0000
Pairing (Intercept) 716.102 26.7601
Residual 89.364 9.4532
Number of obs: 113, groups: Subject, 73; Pairing, 61

Fixed effects:
Estimate Std. Error t value
(Intercept) 58.428 12.255 4.768
Type 7.926 2.852 2.779
Game 15.374 7.147 2.151
PgvnD -8.466 7.554 -1.121
Asym -12.167 7.583 -1.604
PgvnD:Asym 26.618 9.710 2.741

Correlation of Fixed Effects:
(Intr) Type Game PgvnD Asym
Type -0.335
Game -0.835 0.009
PgvnD -0.098 -0.038 -0.010
Asym -0.330 0.081 -0.015 0.189
PgvnD:Asym 0.189 -0.267 -0.011 -0.766 -0.328