A solar panel is not a constant voltage, or constant current source. It can be thought of as a constant power source with maximum rated voltage and maximum rated current. The power is relative to the light hitting the panel, the voltage is maximum with no current, and drops as current is drawn from the panel.
If you are using a 10W panel, and it's in its full rated sun exposure, you'll get 10W out.
If you draw 1A in that situation, the voltage will be about 10V. If you draw two amps, the voltage will be about 5V.
If your battery is full, you probably aren't going to draw much current, so the voltage is higher.
If the battery is nearly empty, it will draw a lot of current, and it will cause the panel's voltage to drop.
In your specific case, what you're finding is that the panel can't provide full charging current all the time - whether that's due to less than full sun exposure, or a low-charge battery depends on the situation.
However, you can still use this system, even though the voltage is low. If you disconnect the battery and measure its voltage, then connect it to the charging system and measure the voltage at the battery, you'll find that the attached voltage is higher - the battery is accepting current from the system, and is charging. It isn't charging as fast as it could be, but that's due to the panel's limitations.
If you want to learn more about this, and what professional solar charging systems do in order to handle this effect, do a search for MPPT circuits - maximum power point tracking. The solar panel is most efficient at a certain voltage and current for a given sunlight input, and these circuits attempt to track that maximum point so you get as much power from the panel as possible.
Also, note that SLA batteries are very forgiving. It may be that you can eliminate the voltage regulator, and just use the diode in the circuit. This will increase the voltage at the battery since the regulator drops 1.5V-3V depending on load, and thus charging efficiency. Given that you're having a hard time keeping it charged, I'd expect the solar panel is unlikely to damage the battery, but check the panel's maximum current at 7.2V and see if the battery can accept a constant trickle charge of that rate.
It doesn't sound like you need anything more between the solar panel and the battery than just a Schottky diode. Your panel is rather wussy compared to the battery, so it doesn't look like there is anything it can do to hurt the battery.
How much current can the solar panel deliver at 13.6 V or so? That is usually the voltage that "12 V" lead-acid batteries can be float-charged at indefinitely. The panel probably can't even put out enough power to get to 13.6 V, especially with a diode in there.
The real problem may be that the panel can't produce enough voltage to charge the battery all the way. Check the panel and battery specs carefully, and don't forget to consider the voltage drop on the diode.
Best Answer
You must advise battery type and specification.
eg lead acid or NiCd or MimH or LiIon or ...?
How many mAh capacity?
You will not harm the panel by charging a lower voltage panel. The battery will "clamp" the panel output to the battery voltage and supply whatever current it cam.
You should add a diode between panel and battery to prevent "backflow" when panel voltage is lower than battery voltage. You do NOT need a regulator for basic charging except if the battery is so small that it will not toilerate a 300 mA charge rate - which is unlikely. You MAY need a regulator to stop the battery from overcharging.
The panel will supply about 300 mA into the battery in full midday sun when pointed at the sun and less or much less when cloudy, not midday or not pointed directly at the sun. If the battery has capacity below about 2000 mAh it MAY be fully charged in less than one day. If 2000 mAh it will not fully charge in one day from "flat".
Where are you located? What use will be made of the battery (daily discharged fully or ...?) These factors affect the answer.