I'm not a doctor but from what I hear all these meds do is slow the progress of
the disease, they don't stop it. Maybe you could call her doctor and discuss
your concerns?  

When my mom gets worse I try to figure if something is going on to increase her
symptoms.  Sometimes an illness or emotional stress can temporarily increase
her symptoms.

Is it near the anniversary of the death of someone she loved?  Does she have a
cold, hayfever or a bladder infection?  Is she aware of her condition and
becoming depressed about it? Is she drinking caffeinated coffee or cola? Is she
bored or isolating herself?  Is she listening to news about terrorist threats?
Did someone in the family lose a job?  Etc. etc.

Sometimes resolving a problem can get my mom back on track.  If she's been more
symptomatic sometimes I ask if something is bothering her and try to get her to
talk about what might be getting her anxious or depressed, and that can help a
little.

Still, I'd call her doctor if I were you and discuss your concerns.

___
"How do they know the dog food is any good?  Who tastes it?" -- Elaine,
"Seinfeld"

as rose pointed out, these drugs dont cure the problem, they just slow its
progression down and in soem cases make the sufferer better for a while
before the inevitable decline continues.
So your mother getting worse is likely a symptom of the diseases
progression. The only respect that I have read about namenda(aka memantine)
making things worse is that if they have gotten bad enough that they dont
cause problems through being argumentative, constantly asking questions and
so on, then if there is a slight 'improvement' they might go back to that
stage.