SPSS - ANOVA question

Hello. If anyone could help me out and offer any insight, I would be totally grateful!

A repeated measures T-test for each group showed one group is signficant and one group is NOT significant. So why would my mixed ANOVA test show NO Group signficance (main effect) and NO Time*Group (interaction effect) signficance?

More details: I ran a mixed ANOVA with:

Between-groups variable = Group (2 levels)

Within-groups variable = Time (2 levels)

Time (main effect) is significant

Group (main effect) is NOT significant

Time*Group (interaction effect) is NOT significant.

HOWEVER:

1-way repeated measures ANOVA for Group A, is significant. Paired Samples T-test for Group A is signficant.

1-way repeated measures ANOVA for Group B is NOT significant. Paired Samples T-test for group B is NOT significant.

If the 1-way repeated measures ANOVA is significant for Group A but NOT Group B, then why for the original ANOVA is Group (main effect) NOT signficiant and Time*Group (interaction effect) is NOT significant? What are some possible explanations?

Time 1 / Group A: Mean = 5.9, N = 18

Time 1 / Group B: Mean = 7.3, N = 19

Time 1 / Total: Mean = 6.6, N = 37

Time 2 / Group A: Mean = 5.0, N = 18

Time 2 / Group B: Mean = 4.8, N = 19

Time 2 / Total: Mean = 4.9, N = 37

Thank you so much in advance for any comments or suggestions you might have!!!