Consistantly low gravity

I am new to brewing and all 3 of my brews have had lower than expected OG and FG. My first 2 brews were an irish red ale. Target OG was 1.056 and both times I got 1.046@71*. Temp correction puts it at 1.047. Grains were steeped at 170* for 45 min as per the instructions which I later found out was to hot. It was a full 5 gallon boil. I started with about 5.5 and ended at 5gal. It has 6lbs LME. I did not weigh the extract myself as I just trusted the local brew shop. I cant figure out whats wrong. I am using an electric cooktop which takes forever to get it up to a boil. Could the longer time change the sugars that are left over somehow?

was this a partial mash or a extract with specialty grains? 6lbs lme in 5 gallons gives you an og of 1.044. If its not a partial mash your target gravity is probably wrong. Next time you could add a pound or two of dme to bump up the gravity if you want.

Are you using extract with grains for steeping or a partial mash? If it is the former, temperature doesn't much play into it- it's a matter of how much extract you have and how much water it is in. If you post your recipe and process it would help.

It is an extract with specialty grains recipe.

10oz 20* Crystal Malt
6oz Munch Malt
1oz Chocolate Malt

6lbs Pale ale extract

3tsp Burton salt
2tsp Irish Moss

Grains were steeped at 170* for 45 min. Next I added the extract and turned on the heat. While heating up I added the Burton salt. During the boil added hops according to schedule and Irish Moss at 40 min in. Turned heat off and cooled with a wort chiller down to 71* and transferred to primary fermenter. Pitched the yeast and took the OG.

The gravity potential of fermentables is measured in points per gallon. This is expressed as the non-zero digits to the right of the decimal point. So, let's take a light pilsen syrup as an example. PPG for pilsen LME is ~36 or 1.036. So, at 5 gallons, if you had 5 lbs of pilsen LME, you would be at 1.036. You used 6 lbs of LME, so you have to divide 36 by 5 to determine how much that will raise your OG. 36 / 5 = 7 rounded down. You are now at 1.043. The grains added were probably somewhere in the 1 to 2 pounds range. If we assume 2 pounds of something common like Crystal 40L, that's about 1.005. So, for a 5-gallon batch, you would be at around 1.048, which is roughly where you ended up. With a 4.5-gallon batch, however, you would have been at around 1.054, which is much closer to where you should have been, according to the recipe.

With an extract recipe, you are in a great position to make adjustments, since you are not concerning yourself with all of the precision of an all-grain brew. Do a smaller boil, get your wort down to pitching temperature, then take a gravity reading and add pre-boiled/cooled water to get your wort to the OG that is correct for the beer you are making. You can always add water, but you cannot take it away.

I would also recommend checking the anticipated OG of the recipe against style guidelines to make sure it is not way off. Better safe than sorry.