#Between the prison attack and Terminus:
##Why did Maggie seem more focused on finding Glenn than Beth?
Because she thought she had some idea of where Glenn had been seen last and where he was heading; she didn't know anything about where Beth was, or whether she was alive. Sequence of events:
###S4E8, Too Far Gone:
Maggie and Beth are near the bus when everyone is about to flee the prison. Maggie tells Beth to get the kids on the bus, then runs off to find Glenn.
Maggie finds Glenn.
She gets him on the bus, realizes Beth isn't there, and runs off to find her.
She asks Sasha and Bob if they've seen Beth; they haven't.
As she's talking to Sasha and Bob, the bus leaves.
Sasha, Bob, and Maggie run away, presumably after deciding that Beth has left. After they're gone, Beth appears, finds Daryl, and says she's been looking for Judith.
Beth and Daryl see that everyone else is gone, and they leave.
###S4E10, Inmates:
Maggie decides to leave Sasha and Bob, armed only with her knife, to find Glenn and the bus. She says:
Maggie: The bus got out. Glenn got out. I'm gonna find him.
Sasha: Maggie, with any luck, the bus is gone.
Maggie: It was heading east down the main road. If I follow in that direction, I might be able to pick up its tracks.
Sasha: Alone? With just that?
Maggie: I'm out of ammo, so, yeah. I couldn't find Beth, but I know Glenn got out and I know which way he went. I'm gonna go get him and I'm gonna come back for you. We both are.
Of course, Sasha and Bob don't let her go alone, and they all follow the road the bus was on.
Maggie, Bob, and Sasha find the bus, full of zombies.
They kill them all, and Maggie checks the bodies and sees Glenn wasn't aboard the bus.
As we now know, Glenn had gotten off the bus before it left the prison, and stayed there for a while after everyone else had left. Maggie, however, didn't know this. All she knew was that she had some information about where Glenn should be, and she had no such information about Beth.
#Between Terminus and finding Beth:
##What they know:
###They have###As Maggie said, "I couldn't find Beth, but I know Glenn got out and I know which way he went." This is why it didn't make sense to look for Beth before reaching Terminus.
###After Terminus, they still had no information about where she might be, and if they decided to look for her, it would take them at least 50 years to search every building in the state. While they're looking, they'll run into over ten million zombies, and they'll all be dead before they've searched a meaningful percentage of the state. Maggie loves Beth, but not enough to get 16 people killed on a 50-year-long search of the entire state of Georgia.