Solar charging question

I have a G103 SL with a 28Ah SLA main battery and a Rotax 505A engine.
Over the years I've had some intermittent problems with ground
starting the engine, and the pattern has only recently been more
obvious to me. The engine has a reputation for being a bit finicky to
start, however, air starts are nearly instantaneous and reliable, and
windmilling alone will bring it to life without fail. My starter has
been overhauled recently, and first starts of the year after having
been on the AC charger in the trailer are usually crisp. I keep the
aircraft tied down and covered during the more mild months of the
year, but this is usually when the engine gradually gets a bit more
challenging to start. I have been told that the Rotax uses a CDI
ignition that is sensitive to minimum threshhold RPM levels, and I'm
beginning to suspect that the battery is not being fully topped off.

I've had a solar trickle charger set up that thought would work, but
now I'm not so sure. I use a Powerfilm rollable R15-300 ( 0.3 A, 15.4
V ) with a Morningstar Sunguard controller. The reason I use the
rollable is that I can bungee it onto the top/side of the glider so
wind doesn't become too much of a problem since it shapes itself to
the fuselage and has a fair approximation to the angle needed for
direct sun exposure.

I bought a new battery last year, but this doesn't seem to keep it
topped off enough to get the full cranking RPM I need. Before I start
blaming losses in the charging system, I was wondering if any of the
electrical engineering types might have some words of wisdom about the
adequacy of my setup. The 0.3 A is only achieved in full sun with
direct alignment, and given the curve of the Powerfilm on the fuselage
and the periodic cloudiness of the Pac NW I'm wondering if this setup
is just underpowered. Even after several weeks of summer sun I usually
find that the battery is only about 12.4 volts or so, and not at the
12.7 volts I need. I haven't found much on the web about solar
charging batteries of this size.

Alternatively, are there other strategies for tie down charging such
as DC to DC or inverters to enable the AC charger? The battery is not
easy to replace, so swapping it out is a bit of a hassle, but
possible. If nothing else, I thought this might encourage a thoughtful
discussion on battery care and maintenance in the field. Any words of
wisdom?