My Furnace In A Condo -

Have worry-free - had 2 visits by PSE&G - with this cold weather I set my T'stat at 69 but the heat never reaches that point. This AM was 66 in the house. Yesterday morning temp read 61. Last Friday tech came out and said furnace was fine- but just wondering why it won't go up. He said it had a 150 degree limit so the flames go out and then 1-2 min later come back. He said to leave it set to 69 all day and night- since Friday had done that. You think the fan speed should be increased so I get more heat in the place. I am on a slab (lower floor of condo)--- this never happened any other winter. Could it be that its been so much colder that the condo is not holding any heat. Have plastic on some windows, towels by door etc. Also wondering if the duct work is blocked in some place. Thank you for any info or help. Tech was very nice and informative..... but this is, like I said, the first time this is happening.