here some more info that might be usefull to solve this problem;
Anode is about 98% pure silver 4cmx 8cm x0,75cm
Cathode is a titanium plate 6cmx12cmx0,3cm
The cell is about 15cm tall by 10cm large
Anode and cathode are about 8cm from one another
The electrolite is made of 90g of silver disolved is 100ml of 70% nitric then dilluted with water to 1.6L
The alligator clip are hooked on 1/4 inch welding rod
Anode and cathode are suspended to the rods by steel S shape hook
Sorry, but I can't think in centimeters. The anode is about 5 sq.in. on the one face that is facing the cathode. In the Thum cell, the most current used is about .35 amps per sq.in. (50 A/sq.ft.) of anode area. For 5 sq.in., this would be about a max of about 1.75 amps. If you go much higher that this, it will take more voltage. A voltage of 4 should be OK, but I wouldn't go much higher. It sounds like the anode size might be the biggest part of your problem. If you do increase the anode size, don't exceed the surface area of your cathode.
If the tank is round with a 10 cm diameter, it contains about .25 gallons. If it is square, it holds about .3 gallons. A Thum cell runs about 6 to 7 amps per gallon. Therefore, at the top end, you shouldn't run more than about 2 amps. If you exceed this, the solution will probably get too hot and this could cause problems.
Everything else seems OK, except, I don't like the s-hooks. They move around and can continually semi-break the connection. Also, it only takes a speck of corrosion to lose contact. It might help to take a file and carefully file a knife edge on the hook where it contacts the rod.
Considering your solution volume and anode area, I am not surprised at the A/V ratio you're getting. If you want to run more amps, I would get a bigger tank and use more anode area.
BTW, at 2 amps, you should dissolve and deposit about 8 grams of silver per hour.
Here's something else I just thought of, although it might not be a problem for you. In my cell (a horizontal Thum cell), I used unbleached muslin, under the anodes, to trap the anode insolubles and keep them from contaminating the crystal. Due to the sizing in the muslin, it was often several hours before I got the full amount of amperage. The sizing sealed the cloth and blocked the current flow. I just started the voltage between 3 and 4 volts. At first, there was very little current flow. After a short period of time, I got some amperage. As the amps increased, the voltage decreased. Every once in awhile, I turned the voltage back up to 3 or 4. After 3 or 4 hours, the system stabilized.
In the same line of thinking, if the anode cloth you're using is too tightly woven, it will impede the current flow. As far as muslin is concerned, I'm thinking 100 threads per inch is recommended. Your cloth should be of a mesh size that prevents the flow of any garbage through it, yet allows the unimpeded flow of current.